Well, first off, both have little car damage, but enough for most. Only nfsc has night driving, and nfsmw has all day driving. In nfsmw, there are tons of police chases that you have to do. In nfsc, there are no forced police chases, but you do need to do them if you want to unlock things. But, in nfsc, there a lot more cars with classes. There aren't as many cars in nfsmw, but the majority of them are sweet. As for online gaming, ive only played nfsc, and it is only ok.
nfsmw-pros-cop chases, better story, better world
cons:no drifting, limited cars, harder(if you don't like a little challenge.
nfsc-pros-easier, more cars by far, has drifting, online is ok
cons-story kinda sucks, world looks similar wherever you go, crewpeople don't help
overall, i would get Need for Speed Most Wanted because the game is a challenge and has a better world and storyline.