Most of the comparison vids will show Audi R8s, but they are 4WD, we don't really see any behaviour in the vids other than slight understeer, but the guy driving is not an idiot, he keeps the understeer in check so it looks like they both have the same sort of physics.
The ONLY way you can compare the physics in each game is by playing both yourself with a variety of different cars. I've only used the R8 and the 911 race car in the Forza Demo and to be honest i don't use the R8 in GT5P and theres nothing to compare a 911 race car with...
I want to know if FF cars get understeer and wheelspin whilst accelerating around corners. I want to know if the back end of some of the harder to control FR cars slips out uncontrollably if you push it too hard. I wasn't able to establish any of these when playing the demo, but you experience them all in prologue.
I have also not see the effects of going full throttle out of a corner in Forza 3. The R8 and 911 race car had exceptional traction because one is 4WD and the other has weight that pushes the tyres onto the road for rear grip, so i could'nt tell this from playing the demo either. And most of the other vids i have seen don't give the impression that you get any wheelspin in 2nd and 3rd at all, but again its down to the driver, if you watch a vid where a car is driven by a skilled driver, you see a car going around a track, thats all. It tells you nothing about the physics.
As for the graphics, both could be better, but to put it this way, the best shots of GT5P are a lot better than the best shots of Forza. But you could find images of both games that are equally bad. Basically i am saying GT5P overall is better, but there are going to be some shots of Forza better than the bad shots of GT. But the best shots of GT cannot be matched.
Look at the rest of this post too:
https://www.gtplanet.net/forum/showpost.php?p=3609063&postcount=68