I like both franchises now. There's things I like better about GT5 and there's things I like better about Forza 3.
I like the graphics way better in GT5. I feel its just way nicer looking when you're actually in game, in a race. Forza 3 looks okay, but I'm really hoping Forza 4 looks at least as good as GT5, if not better.
Pretty much everything else (besides graphics) I liked better about Forza though. It has better car choice IMO, it has a lot more tuning options, it has more options for difficulty changes to make the game harder/easier (plus you earn more money the harder your settings are). Also the levelling up system is pretty cool.
One thing I hate about Forza is when you go to sell cars the game only gives you a couple hundred dollars even if its a friggin lamborghini or something. The only way you can actually sell cars and get decent money is to sell them online, but nobody wants the shit cars you'd be trying to sell. And in GT5 I hate how you can't sell all your cars either. Like half the cars I have when I go to sell them it says "this car cannot be sold, remove from garage?"...and that's just stupid.
I'd give GT5 about a 9.2/10 and I'd give Forza 3 about a 8.8/10. If Forza 3 had GT5 level graphics it would easily get a low-mid 9's rating from me.