FORZA!
Definitely!
In Forza, A LOT of upgrades affect the sound of the car in the end. (most noticable change would be the cams as they completely change the way the car sounds and partially behaves.
I have to disagree with Diablo on having a better feel with the Wheel (hue, rhymes D:) on GT than on Forza.
I had the PS3 and GT5 for a while, I was using an equalizer and amplifier to get better sounds, but they were still bad. I had to get used to driving with the bonnet cam because I either go all cockpit or all bonnet as it changes how I drive (personally).
In GT5, when going in a straight line, you could launch in fourth gear with a Corvette ZR1 and nail the accelerator to the ground and the Corvette would simply not spin out, but the wheels would spin and spin and spin. Get in first gear with a Forza Corvette, nail the pedal, power shift into second and keep the accelerator pushed and you'll instantly spin down.
A huge turn down on me.
Another, even bigger turn down was the X1. Seriously? "The best driving simulator"
Yeah, with a 400 kilograms, 1500 HP hyper F1 car? No, thanks.
In GT5, the only mistake I could do is brake too late or spin out in a corner. In Forza 4, a high powered car is way harder to control and I have to grasp on my wheel and drift around, eventually resulting in me, spinning out (I like to go through corners with an ape-lalalala-speed to test the car's limits).
Overall, Forza just seems more realistic to me.
Better sounds (fact), better customization (fact), better physics (opinion)
GT5 wins in terms of track accuracy and looking better to the eye, because Forza just looks like we would want to graphically see whereas GT5 looks close to real life (IMO)
Did I mention that GT5's carbase is around 80% Japanese cars and of those, around 30% Nissan? :P (along the lines of)
Also, sitting on a game for THAT long and presenting us 200 premium cars and the rest as standard cars with PS2 models is really, really wrong.
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By the way, does anybody know if GT6 comes out for the PS4? Seen the concept trailer and I could swear it said November 2013 (PS4 release - ??)