The lighting engine is graphically better, while the car and track models are about the same if only slightly better. However, I personally like the improved physics and tire modeling that they used. The updated sound engine makes the cars sound more visceral, and it's a much more engaging experience.
The added Rivals mode is nice if you like to race against others without necessarily racing online (if you've played any recent NFS games, it's basically just like autolog). Autovista is cool the first time you get to use it and unlock new cars, but I have probably only looked at all the cars once or twice ever.
The easiest way to put it, I haven't touched FM3 since I got this game. Unless you just want to drive Porsches, which I hope will be rectified after EA and Porsche's license agreement expires, there's no reason to go back.