NFS MW is ok, but its crap when you compare it to NFS carbon. The graphics are a million times better and there are absolutely loads of customisation options avalable (inc. the autosculpt). NFS MW is a bit too easy to progress through imo and only offers a challenge at the very end of the game where you need to evade cross for 5 mins and then make that jump - that was kinda hard.
Carbon also has lots more replayability (some of the reward cards will keep you going for ages) and more online modes. You won't get 1000 points here in a few hours!
My favourite part of carbon is the addition of the new race type where you need to race to the bottom of a canyon. It's so intense (even against the computer). First you start off my chasing the computer down to the bottom and get points for keeping up with them. You then switch and the computer chases you to the bottom, where you lose points as the computer gets closer. If you finish with any points remaing you win.
I'm just on the final boss currently and would definately say it amongst the best 360 games i have played so far.