NWN is not at all that much like Diablo. Diablo has a strong focus on flashy abilities and combat in general.
NWN is much more of an RPG. A lot more quests, decisions and conversations.
Neverwinter Nights - Wikipedia, the free encyclopedia
If you decide to get it, I would get the diamond version yes. It's a lot more gameplay and even gives more choices for the OC.
Anyway, you'll have to tell us what kind of games you like. I wouldn't buy NWN, because you like Diablo.