I have BOTH systems...and like both. Forza may in fact be better than GT4, but I personally doubt it. I think that Microsoft just wants in on the GT marketshare.
I mean how many times have we heard "this will be a GT killer" only it turns out to be VERY subpar like Sega GT or APEX(lol). Even if it is, that doesn't mean I'd stop playing GT (doubtfully). I've been playing Gran Turismo since 2 (yeah, I actually never played the original) and I don't plan on stopping.
The thing about GT is...it's one of the few current series of games that I like, so I wouldn't really want to give it up. It may be blind fanboyism, but I'm entitled. The difference is, I won't go around saying "XBOX SUCKS" everyone is entitled to their opinion, just don't go throwing it out as fact.
Oh and as for GTR, which was mentioned earlier in this thread...it may be a better simulation, but that's it. The thing about Gran Turismo is, it has a LOT of different kind of cars. GTR only has GT cars. I'm not saying in any way will it be a bad game, but it's much different, and I may buy both.