As many have said GT5 isn't out yet but here's the facts...(with some opinion)
Forza 3
More real-life tracks
Less cars, 400 which are all equal (so all can be upgraded etc)
Decent sized Career mode (opinion)
Good graphics but not on the same level as GT5 (opinion)
Not as many in-depth tuning options/upgrades
No real wheel support (opinion)
Physics which resemble more of an arcade game (opinion)
Online- 16 players
No weather
No rally (apart from tarmac)
GT5
1031 cars- only 200 can be upgraded and have cockpit view- the others are 'standard'
Fantastic graphics (opinion but I'd hope you all agree)
Less real-life tracks
Good wheel support
Good tuning/customisation
Physics- Unknown but (opinion) looking promising
Dynamic weather
Head tracking
3D support
Rally
NASCAR (with pit-stops and sparks!)
Night racing
32 player online (16 racing and 16 spectating)
That's just my little overview but really it's up to you. Sure, Forza 3's been a great game for me, really enjoyable, but now come GT5 I'm moving on.
If I were you, well, I think you know the answer. When you look at it, GT5 has a lot more in terms of features than Forza 3.
Also this:
Then PD got their act together. Past GT games were the same as what you critique Forza for.
Flat surfaces was probably the most offputting thing about GT in the past and I really hope they've sorted it out for good.