Now, this thread is kind of stupid as I'm pretty sure the world is big enough for both Forza and Gran Turismo. But just for the sake of it, I'll share my opinions on the matter.
Forza:
Well, I have only played Forza 3. Only seen videos of Forza 4 so far and that stupid "review" on IGN (was that review for real? I was expecting a serious review of the game and instead I got someone reading a script written by Turn 10's marketing division), so as a judge I'm perhaps not the most educated one on the matter.
First of all, the paint shop in Forza (3) is for me the absolute highlight of the game. I love to recreate cars from various racing series. Something that could be improved however (and maybe this has been done in Forza 4?) is to allow graphics to be put on windows and wings as well. Some wings can be painted and stuff, but not all. I appreciate ho easy it is to work with paint jobs, given the fact that the main tool is the default hand control, extra points for that. However, more advanced graphics can be quite time consuming to create. I give the feature 8/10, because it's so nice that it's there, but there is room for improvements (based on Forza 3)
Next topic is car selection. It's the second best thing. There's a good balance between low powered and high performance cars. Even got some nice exotics, like Koenigsegg. Personally though I'm a fan of classic cars and cars with unique personalities, and in that respect at least Forza 3 had some catching up to do. Maybe Forza 4 is better there as well. I'd give the car selection 6/10.
Next topic is track selection. I'll be totally honest, I'm not a big fan of the tracks that are originally designed by Turn 10 themselves. To me it seems like they put too much effort in creating a spectacular scenery, that doesn't seem very real, and too little effort in making the track actually enjoyable. Like the Fujima Kaido for instance, really spectacular scenery but the road doesn't feel very real. In terms of track layout that specific track does not have so much variation either. The Turn 10 designed tracks only scores 3/10 in my book.
The tracks from the real world is a different story. Just the fact that Nordschleife is there is enough for me. 7/10 in score for that. Which gives an average track score of 5/10.
Graphics. I thing the field of view in Forza 3 is really bad. Has been fixed in Forza 4 though, so not a problem anymore. Rest is fine I suppose, the graphics are up to par with the general standard of this generation of video games. Extra points for the sun glare effect and the change of time effect in Forza 4, that's a huge improvement. Big minus points though for no night racing and no weather change. Autovista graphics seems out of this world, awesome. The graphics rendered during races seems to be mostly as they were in Forza 3 though, which again is totally okay and I didn't expect anything else. When it comes to my personal taste I'm not the biggest fan of the actual graphics art. Feels heavy and dark, too much of an FPS. It doesn't make the graphics bad, but it's not an aesthetic that I like. And as I am the one awarding the points here the result is a 6/10 for the graphics.
Damage:
Damage is okay. The mechanical damage is fine, I appreciate that slamming into cars and walls makes bad things to your car. In Forza 3 it felt a bit generic though. The car always behaved the same when you had sustained a certain damage, some variation would be nice.
Cosmetic damage is okay too I guess. Most arcade car games have better cosmetic damage than Forza, but it's good enough for me. Damage score: 5/10
Controls:
Now, Xbox 360 is supposed to be supporting a wheel again. That is great news. Bloody bad though that there's no standard that is supported by Xbox, that it always have to have some unique solution that only works for Xbox. If my computer can support the Logitech DFGT wheel, why can't Xbox?? Huge minus score. Another huge minus score is based on my experience of Forza 3 and the default hand control, I think it is really bad. I don't feel that I'm driving a car when I play Forza 3 with the hand control, it feels too much like an arcade racer. It's better than Mario Kart, but not by far. Maybe it gets better if you play every day and are familiar with how the controls work on your car, maybe it's easier to make precision steering then. But I'm not there and thus the score for controls is a devastating 2/10. I hope this has shaped up a lot in Forza 4. If not, I suspect I'll be mostly painting cars.
Gran Turismo
Now over to Gran Turismo. GT was not the first racing game I played. The very first racing game was probably NFS way back in the mid 1990's. The first proper racing game, which could arguably be called a sim, was a NASCAR game and an INDYCAR game for the Macintosh computer, don't remember who made those games though. Then in around 1998 or so I got Gran Turismo. It was a nice game and I played it a lot. Don't remember too much from it more than the HiFi mode, a Toyota Supra race car and the song "Sweet sixteen". Then I got Gran Turismo 2 and that really got me. Never got the PS2 though so I've always followed the GT series sort of from the shelf. Then when the PS3 came I bought it (mainly because I needed a DVD player and the extra bonus of getting one that could also play games as too much to resist). I bought GT5 Prologue too see what had happened to the franchise scince GT2 and the first thing that struck me was the graphics. Watching replays from High Speed Ring was an amazing experience. So that was when I fell in love with the GT series again, after not playing GT2 for probably 5 years or more. Now, I didn't give a background story like this to Forza and frankly it's because I don't have a personal relation to the Forza series. For me, Forza is a game and nothing more, while the GT series for mainly nostalgic reasons have a deeper meaning. Forza is like "nothing personal, only business" while GT is as close to a love affair that I suppose you can get when it comes to a video game. Will this make me biased? Yes, and that's why I think it's important that I am honest about it, because if I'm gonna pretend that I'm being neutral in this matter I'll end up like that IGN reviewer who sounds like he is reading a Turn 10 script while trying to convince his audience that he is unbiased.
So, for GT let's start with it's strong points. First of all the graphics. Now there is a big discussion on standard versus premium so I'm just gonna divide this section in two parts for that reason. One premium part and one standard part. First of all the premium part: It is amazing. Now it is not very even, for instance does all tracks not have weather or time progression. But those tracks that has both are by all means the state of the art. I mean, it's so far beyond "on par with this generation of video games", in fact it's probably on par with what the next generation on video games will be like. "What about the shadows" someone says, and yes, they look really bad at times, but I don't think that is really important. The important aspects of the graphics, from my point of view, is what you see when you are racing. If you have time to look at jagged shadows, pixelated edges where a smoke effect meets the body panel of a car, two dimensional trees, then you are not racing. Or if you are you are gonna end up last. Shadows would be better if they weren't jagged, but just the fact that they are there is plenty enough because with everything that happens during a race that is all the eye have time to register. The graphics aren't perfect when you look at individual things, but altogether and in motion it's pretty amazing. A race around Nordschleife in the night is the state of the art when it comes to this generation of video games, no question about it. It's a solid 10/10, partially because I'm biased and partially because it's that bloody good.
Standard then? Well, some cars do look very outaged, that is true. During racing the difference becomes a lot less apparent though because everything is in motion. Still well below this generation of video games that is drawing the total graphics score down to 8/10. DO REMEMBER THAT I HAVE ACCOUNTED FOR THE STANDARD GRAPHICS NOW, okay?
The second strong point is the car selection. It doesn't have everything, but +1000 cars in GT5 is quite amazing. The emphasize is on the Japanese market, that is true, but then again, the Japanese motor industry is one of the biggest in the world. "Lots of them are simply duplicates", someone says. Well, no. There are a lot of Skylines in GT5 that is true, but all of them are different versions. The difference might be subtle in some cases, but if you look at the real motor world you will find that there are plenty of subtle variations there as well. It doesn't exist only one Mazda MX5 model, there are probably hundreds of variations to them. Far from all are in GT5. And so what if there are more than one version of a car? You are not gonna drive a thousand different cars anyway, so why not have the choise to select the special one model that YOU prefer? "They could spend the time making other models", well yes and no. If you're gonna make five different R34 Skylines where the only difference is some aero parts and engine displacement, then you only need to make one base model and then make small adjustments to that. It's a lot quicker than to make a whole new car. To convert the time spent on making the "duplicates" into making new models instead would probably have a ratio on 1:100 or 2:100, so in reality we might be missing out on two or three cars at the most because of the "duplicates". Not a lot if you ask me. Now, a thousand cars with everything from Kei cars to LMP's and even the X2010 and X2011. You can have a Kei car cup and there are 16 different cars on the track, you can have a 4wd sedan car cup and there are 16 different cars on the track, you can have a Mercedes event with 16 different cars on the track, you can have an LMP race ith 16 different cars on the track. With the latest update it's even possible to have 16 different NASCARs... And as I'm personally a huge fan of 1960's cars I love that there's a lot of them, and that most of them are Japanese doesn't bother me at all. And as we've already accounted for the graphics this gives a car selection score of 8/10.
Track selection: As I am an old GT2 player, the original GT tracks do have a sentimental value for me that the Forza tracks doesn't have. I like the new tracks as well, especially Cape Ring which is very technical. Not a big fan of the rally courses, even if some of them are spectacular. Original tracks gets 5/10 (there could be a lot more of them) and real world tracks gives the same as for Forza, a 7/10 (as we've already accounted for the graphics, not even Nordschelife at night can award bonus points here) which gives a total track score of 6/10.
Car customization. I'll be honest. It's not very good. Paint chips are nice I suppose but very annoying that they disappear when you use them. Aero parts for some cars are nice, but there could be a lot more. Can't change rims on standard cars and can't add graphics to any car... It's kinda bad. I give car customization 2/10, it can be improved a lot.
Damage. Well, cosmetics are well below Forza, that's for sure. Mechanical damage is there, but not as detailed as Forza. I can't give more than 2/10 for that. Then again, damage is not the key aspect of a racing game, but it can be important for cultivating a realistic driving behaviour.
Controls. I have the DFGT wheel and I love it. It was hard to convert from the DS3 in the beginning but now when I've learned how to use the wheel it's simply fantastic. The car does whatever I want it to. It doesn't feel like I'm holding a controller, it feels like I'm in the car. It's an easy 8/10 for that.
Now, total score:
Forza (mainly based on Forza 3, plus videos and reviews of Forza 4)
Car customization: 8/10
Car selection: 6/10
Track selection: 5/10
Graphics: 6/10
Damage 5/10
Controls: 2/10
= 32/60, not bad
GT (mainly based on GT5)
Car customization: 2/10
Car selection: 8/10
Track selection: 6/10
Graphics: 8/10
Damage: 2/10
Controls: 8/10
= 34/60, also not bad
Now, there are plenty of other variables to consider, but I can't waste more time on this now so it will end with this. I will continue to play both Forza and GT, for different reasons. The world is big enough for both of them.