I'm a fan of both and they're both good games in their own way.
Diablo II is 99.9% combat. Quests involve killing monsters or retrieving an item guarded by monsters who need killing. It is entirely balanced and designed for combat, and it is very well balanced.
Neverwinter Nights is more of an RPG. The original campaign is a lot of hack-and-slash, but there is a massive library of mods available. There is a lot more interaction and storyline in this one.
So really, it comes down to what you feel like playing: Action or Adventure?
