OK, time for some honesty.
Who owns both GT5 and FM4, and if you HAD to pick just one of them, which would it be?
For the record: I own both, I play both, but if I had to choose just one I know I'd get more overall enjoyment, for longer, out of FM4.
I've played both, if that counts.
I'm basically a GT fanboy.
Physics, FM4 > GT5. Overall, GT5 has a very good driving feel. FM4 also feels good. However, Forza has a superior tyre model and suspension model. Furthermore, GT5 feels like the real understeer simulator compared to Forza.
Graphically, GT5 > FM4.
Sounds, hit and miss on both games. Some cars sound pretty good on GT5. A lot sound crap. Forza overexaggerates, but generally is more sound than GT5. I also hate the annoying squeeling effects in both games. Still, due to consistency, FM4 > GT5.
Menus, FM4 > GT5. Fuck this convuluted bullshit design, PD! FM4's menus are simple, clean and elegent.
Menu music, FM4 >>>>>>>>>>>>>>>>>>>>>> GT5. Jazz sucks.
Customisation, FM4 > GT5.
Cars, GT5 = FM4. When you have so many shit looking cars, it doesn't really matter how many you have.
Fun factor, GT5 > FM4. I've put countless of hours in GT5. I looked forward to playing it everyday with my DFGT. With Forza, I didn't really care too much about it when I had my 360. I played it for <20 hours at most. I can't remember really.
So, I would like PD to learn from the physics of FM4. And menus. And menu music. And the sound effects.