i agree with the above posts.
the one caveat i'd throw out is that diablo is a better single player game or PvP, while NWN (as well as the excellent baldur's gate(s)) are best played WITH someone (or two) in a co-operative manner.
if it's single player you're after, don't discount planescape torment.
the 'interest' level will probably last longer in NWN than diablo. diablo II suffers much like dungeon siege and (truly) divine divinity in that you can just become far, far too powerful for there to be any challenge in the hack-n-slash aspects (monsters and bad guys). so you have to hope for good story content (like DD) to propel you to the finish. NWN on the other hand is built upon D&D rules and the treasure is considerably less, thus you don't become too powerful, too quick.