Is stablish that there is a hidden TCS in game,cars like the F40 are not that stable and the fact that the cars are more controllable or have more grip doesn't mean that the game is more realistic.
I been playing both FM3 and GT5 the last two days so physics wise GT5 wins,(you probably will say no and for some magical reason FM3 for you will be better),the reason of why GT5 physics are better is simple,in FM3 you have the options of tire setup and fine tuning(you have to buy the tires but it can be done),and GT5 allows you to do that in pre-race,the thing about GT5 is that there is a larger line-up of tire compounds to test,and in that aspect is were FM3 should beat GT5,but the thing is that FM3 doesn't beat it,it falls back because the tire compound in a car like an F40 or F430 doesn't have that amount of grip,in GT5 there is more tire compound to choose and you eventually come across(specially in the S range)a fine set-up to duplicate real life condition and the do the test.
In my case I took both F430 and F40 over TG test track,(sound are better in FM3 I amid it ,the F430 just sound amazing),but S3 tires in hammerhead looks exactly the same as the show does,also the chassis in GT5 seem to more spot on,it bounces and is noticeable while driving,after that you start to play with the settings and tires,for this I use the same TG test track and Fujimi Kaido,both games feels complete different even if you intent to make them drive in the same way(TCS,ABS,MT-AT,same tires set up),in the GT5 case the car feels a bit more alive,in FM3 as well but in GT5 the cars have more identity,graphics FM3 cars sometimes doesn't feels right,like the engine bay seems like a cardboard box were snake can hide(look FM3 F40 engine bay),and GT5(premiums)just are almost perfect,is hard to find mistakes on premiums(there is a jagged edge behind the pedals on GT5 that's what I found so far).
So contact to the road in FM3 is not realistic because the cars feels glued to the road and that is not realistic in cars like Dodge viper and Alfa Romeo 8C,the only way to tell both physics is just play both games,side to side and then comparing it with a real world video.
And finally against what other people say the standard cars can be good fun,Audi RS6 can be good fun,more fun that in Forza cockpit view because that car feels more free to the road and more alive,something strange because Audis in FM3 are fun but not as fun as GT5 versions(even standards).