Without ANY INTENTION to go into Forza vs. GT debate, ONE MINOR issue stands on Forza's way to be a true simulation - support for true force feedback wheels.
Forza series has some great options, miraculous online, almost perfect collision model, good tire/grip simulation in C,B and A car-classes - but it fails in wheel support, period.
Since I have Microsoft wheel and have been Xbox user since the first day of the first Xbox console - and have spent some hefty time in Forza 2 - it is far the greatest weakness of otherwise pretty solid game.
Microsoft's selfish decision not to implement adequate hardware/software in console - being solely driven by profit reason in such small margin that it's laughable - made their console and one of the most important franchises obsolete in any serious discussion and usage.
Even the semi-official announcement of new Forza 3 wheel - that being Fanatec's 911 Turbo S model - just can't do any good because the above mentioned hardware/software restrictions. I have further elaborate that matter here, so feel free to click:
http://www.neogaf.com/forum/showpost.php?p=15859760&postcount=1
If Microsoft have been less greedy with their ridiculous decision regarding "profile to device" protocols and understood they would make more profit in long term if they go with already established force feedback solution, I can safely say that Forza 2 would probably still be the most played driving-simulation in the world.
But, even Forza 3 will remain heavily compromised in any serious discussion because the mentioned hardware-limitations. Of course, I do not doubt for a second that fanboyism, biasism and internet communities will successfully do thier part in minorisation of such important feature - but the fact remains.
I remain deeply confident that Microsoft have learned what does proper FF implementation can do for both their hardware/software business in such specialized genre such are driving games and that NeXtBox will come wiith adequate FF support.
Until than, any serious discussion regarding Forza vs. Gran Turismo issues ends after the slightest look at my X360 Microsoft Official Force Feedback Wheel.