Whew. Lots of interesting banter going on here. As someone who has and plays (regularly) both games, I find them to be quite equal in enjoyment.
Forza:
Pros:
Consistency: There is no difference in the quality of any of the vehicles.
Audio: Car sounds SOUND better, even though a lot of the lower end cars are too loud, and their aggressiveness is highly exaggerated, it's just more pleasing to the ears.
Online: Lets face it T10 has been at it online for a while and PD is just now trying their hand at it.
Customization: No real need for explanation here is there?
Cons:
Graphics: Depends on who you are. I don't like the fact that even now, after 4 iterations, the cars still have a high gloss plastic look to them. I can't explain it any other way, if you don't agree, then you don't agree.
Physics: I know I will get some beef for this, but I just find Forza to have less resemblance to what I have experienced in real life than GT.
AI: Neither GT nor Forza does this very well. It's either fast AI or cleaner AI. They both still have not figured out how to get fast AND clean AI.
Autovista: Sorry to most of you, but I consider this a con, much like GT's automotive history stuff. I can't race the car with that level of detail, so in my mind that is pretty wasted effort. I'm also not a very big shutterbug.
Race Elements: Lack of weather, and night racing, and limited number of cars on course.
GT5:
Pros:
Graphics: They're better than Forza 4. They just are. I know, I know, "but what about **** standard cars?! Or the tracks that are just ported from GT4, and earlier?!" I'll get to that later. Tick for tick though, GT5's premium cars just look better than Forza 4's anything IMO,and GT5's tracks are better too, and tend to be more accurate, at least I think so.
Physics: I find both games to be arcade-ish in areas, but I find GT5 to be closer to my own experiences when it comes to driving physics, and what actually happens out on the course.
Racing Elements: Weather, night racing, and less limited number of cars on the course.
Cons:
Consistency: I wish that all of the cars had been up to the premium car level. I still like the fact that I have the standard cars to drive (because in the end, that is what the game is about, driving) but I do not like the lack of consistency at all. Especially in regards to the tracks....

Audio: Even though I think it is better than most other people tend to think it is, I still find it extremely lacking in the areas of the higher end cars. The pitch sounds to high, and their filtering system is to clean. If they added a distortion effect to most of their sounds, it would greatly improve the overall sound quality. When the human ear hears loud noises, most of it gets distorted due to the packing of air in the ear canal. Sound waves are physical waves that create differences in pressure. The louder the noise the stronger the wave, the more the air is pushed. GT lacks any form of distortion in their audio, which makes the cars sound tame and quiet.
Online: I love it, but it's not implemented well, and WHERE THE HECK ARE THE LEADERBOARDS?!
Customization: Again, no comparison here.
Photo travel: I also consider this a con. That level of detail on the interiors of the cars has no effect on the racing aspect of things. So I see it as wasted effort. However, most of the detail on the exterior of the cars is still there during the race, that is why I think GT5 beats Forza graphically.
This is all my opinion, take it as you see fit. However, both games are VERY fun, and I have wasted the same amount of time on each. Especially playing split screen mode. I just wish one of them would implement AI on splitscreen mode. If one of them did that, the other would end up collecting a lot of dust.