I prefer NWN. The weirdest thing is that I bought WC3 the second it was in stores, and I got NWN a few months after European release. But WC3 was kinda boring after a while: same units, same heroes, same items, same objective. On the other hand NWN doesn't have any sort of skirmish (which is the only good part in WC3 SP), and you need to replay the OC over and over. Neway, I played NWN longer than I played WC3. A quick list of pluses and minuses. WC3 is a real rushing strategy game: kill as much creatures a.s.a.p. to get exp.
WC3 +
4 races
Soon to be released expansion pack
Mix between RPG and strategy
Can download lots of maps
WC3 -
Gets boring after you've played with all units
Skirmish is really hard in the beginning (read: overwhelmed by a huge army in five mins)
NWN +
Lots of different classes to play, it's like multiple games in one
One exp pack released, another one in development.
Some people here (about 5) play it, so we can play it online sometime
Lots and I mean LOTS of stuff to d/l for NWN (short missions, new creatures, new placables, new sounds, new portraits, etc.)
NWN has its own clankiller forum
Can choose a good path or an evil path
NWN -
No skirmish, just one OC to play (although there are quests only certain classes can have)
Could be that I forgot anything.