Thanks Lambo88 for the comparison, couldn't agree with you more...
About the tuning and modifications, Forza takes the lead... Why???
GT's tuning process is pretty straightfoward; You pick a car, dump the biggest turbo/intercooler/supercharger in it, and you're off..... Forza on the other hand, goes much deeper.... Not every turbo will make your car reach a higher topspeed/acceleration, as not all turbo's can reach their full potential on certain engines, so it will be your choice to make;
for example, will you pick the turbo that pushes your topspeed over 50%, or will you go for the one that gives your acceleration a 50% boost??? Or will you just cope with the one that gives you 25% better topspeed and 25% better acceleration???? Some cars will give you a whooping 75% boost on both factors, depending on which engine is in it.... And if you like the car but not the engine??? Swap it!!!(Although at the moment it's only possible to swap engines of the same make, due to no agreement between the different car manufactuers, but this is being worked on, as M$ says)
Over all that, you can also fine-tune gear ratios, fuel-mapping, forced-induction pressure, tire temperature and pressure, and ignition timing etc. BTW, the cars incur damage and wear, which affects car performance. Advanced tire and suspension models respond to heat and pressure changes, weight transfer and aerodynamic load. Yes people, this game is
HARDCORE!!!!
In mods Forza also owns.... You can add real-world body kits, rims, spoilers, decals, side skirts, fender flares, and hood scoops etc. Plus, these cosmetic upgrades don't just change the look on your car, but also changes the handling (aerodynamics)... BTW cobra, Forza has already got a load of amazing aftermarket tuners, more are to be added later:
Hennessey
Saleen
Shelby
Guldstrand
Lingenfelter
Aerogear
Mugen
Wings West
AB Flug
INGS
Sparco
Mine's
Tommy Kaira
APR Performance
Border
Tom's
VeilSide
VIS Racing
It amazes me how some of you know so little about this game..... Too bad though...
Sorry for the long post!!!