All the fanboys love to argue over GT and Forza, to be honest... GT5... if it really comes out in November... will obviously be better than Forza 3, its had more development time.
I'm sure Turn10 is already working on Forza 4 as we speak. Maybe if Forza 4 and GT6 come out at the same time that will be a fair comparison. But, comparing two racing games with completely different development times and release dates over a a year apart is kind of silly.
I've played Forza3 quite a bit and a lot of the features in GT5 are pretty sweet, does it make Forza any less of a racing game? Nah, it's really fun. I'm sure GT5 will be really fun as well.