Vicious, the point of my post is about comparison. The comparison is null, void. It's garbage. So why bother? Both are games to play when they come out. Comparing the two is like ricer retards screaming EVO X vs STi! Both are great machines, but both will be shitty in their grubby, greasy amateur hands. Don't even both talking about "max speed". It's a fucking straight line. Not hard to drive, and you probably don't have the balls to drive at the max potential speed anyways.
Based on the games...
If someone asks which is better. I shrug and say "just try both, but I personally don't like Forza."
If someone asks which to buy, I'll ask which console they own and tell them to buy for that console. If they have both, I'll tell him/her not to be a cheap fuck and just buy both.
I had great fun with the GT series, so I'll probably get that when it comes out. (buy a pS3 for it too if required, but currently going to buy an xbox elite to "upgrade" my original xbox.
Racing games... I'll go with project gotham or need for speed arcade. Forza annoyed me to no end when I played it before. (I was excited for it too when I read reviews and stuff, but playing pissed me off.) I'm no video game retard by any means, but damn that game was screwy. For my xbox360, I'm pretty sure i'll have some fun. But it won't be with forza.
If you like forza, play forza. If you like GT, play GT. If you like both, play both. But if you try to compare the two, I'm going to call you an idiot and just tell you to go drive a car in real life really fast. And if you argue realism. I'll tell you to do doughnuts in a parking lot and add that I hope you crash into a light post because that's the level of intelligence I'm detecting.