Ok, this is an unbiased review on both, I'm going to be putting my fan-boy side of me aside here, as the PS3 has better hardware, but I'm going to write down a list of what I have seen playing both of these games.
First let me start out with saying I've always been a die-hard GT fan, I had GT1 when it was brand new, same with GT2, I remember how realistic I thought it was when 2 came out, and 3 and 4, and had 5 CE preordered for a long long time. I have always been a Playstation loyalist, it just shuts out any other console (personal opinion) but at least for graphical HD content. It has a better cpu, faster ram, and of course we cant forget the superiority of the size of blu-ray discs. Now with that aside.
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Forza 4 vs GT5
Better Shadows, Impressive Sounds (All cars have unique sounds, and sound different with every route you customize them, Cars with extremely unique exhaust notes like the low grunt of the WRX was nailed spot on). vs GT5 has many, many repeated sounds that use the same sound file.
Customizing (everything that GT has, plus tire temps, thermal dynamics, benchmarking, drive train swaps, engine swaps, more attention to fine detailing in springs and rebound, adjusting rim size tire size/width, lots more.
Full Livery, Auction House, Full Customization of body parts, many many wheel / body part manufacturers.
Now, On to physics..
I agree with the poster earlier on the oversteer/understeer physics for forza, you can feel the tires a lot more connected to the road in forza 4 than GT5. Some of the grip physics are better in GT5, and some are better in forza 4. Depends on what angle you're looking at the opinion from.
The physics themselves in both games are good, I say good because being on par with Iracing or Rfactor or any of the like on a console is a pretty far fetch, but as for console sim racing, they are both very good.
A prime example -
Drafting, even on the lowest setting on GT5, it still pulls you far, far beyond what it should ever do. Drafting in Forza 4 feel's realistic, you dont gain 10mph standing behind a car for 3 seconds, you barely gain anything at all. (I know that GT5 has boost settings, even on the lowest, it's still unrealistic), Also lets compare GT5's top speed on vehicles, there is no SL600 with 1030hp that would draft to 290+ mph, just wouldn't happen. I've been taking notes on small things in both games, example, Forza 4 the "rated" top speed on the factory vehicles match up, 186 mph was as fast as it could get on le sarthe on forza 4, which is it's "actual" documented top speed on the countach LP500s. Versus I can get into a stock car in GT5, factory specs like the honda CRX, and hit 140+ with it on SSR7, that just would never happen on factory specs.
GT5 to me just came out incomplete, even if they are patching it now (I'm glad they did) It was nice to finally see full gear ratios, adjusted physics.
Their first "vehicle" update had many cars that were already in the game just reskinned with different graphics, and a remapped sound file of the others, not trying to be negative, just unbiased, I'm a GT/PS3 fanboy, if I was being biased I'd be leaning towards everything PS3/GT related.
I don't understand the need for 50+ skylines, and Everything else with "multiples as standards that were imports from GT4" It's just, why have that, and then knock the sound quality, put in 3/4ths standard model cars, and other things.
One thing GT5 has going for it is better lighting, and some of the physic mechanics in GT5 are better, and the smoke detail and how it lingers with the wind is better, a lot of this happens to be because of overall disc space, just the install file alone for GT5 is almost as big as forza's play disc. day/night lighting transitions and weather and working wipers on each of the premiums took a lot of the data, and things like track heat waves.
Both games are good, Both games should appeal to all die-hard fans of the automotive industry, Forza has lots of things GT5 doesn't in the car list, if they died down about 20 of the skylines, 10 of the S2ks, and 15+ of the miata's they'd have room for stuff like the syclone, typhoon, Jeep SRT8, Older muscle, more unique auto manufacturers that are featured in Forza 4 like Joss, Morgan, Mosler, Gumpert, Saleen.
It's worth having both games if you are a huge automotive enthusiast, they both compete well in all catagories for car enthusiasts with a side of sim, The physics are good in both. I will always love the GT series, I plan on playing both and buying the DLC for both, I'm glad they both have decided on "DLC on a regular basis"
Both are great games guys, there's no "arcade like" feel to forza, just area's they need to improve on, as GT5 does in a a lot of area's as well, I hope that GT5 takes what premiums they have and gets ready for GT6, and starts over a new list of things to make for GT6 so they don't have looped sound files, jagged shadows, and that they make 100% of their cars premium with full mods instead of the same spoilers on 70% of them they've used since GT3-4, only a few of the premiums have unique body additions.
Last but not least, Forza 4's "Hire driver" drives much better than any of the Bspec drivers, but also on another note, GT5's AI is miles ahead of Forza 4.
That concludes my review of both games.