They're like chalk and cheese. BG2 is a traditional tactical RPG, where you control 6 characters, with a great plot, great gameplay, decent graphics, etc. It's primarily a singleplayer game - the main selling point of the game IS the game itself. Tried and true formula.
NWN is totally different for mine - you shouldn't judge it on the SinglePlayer game (which I think is quite dull, to tell you the truth). NWN is not pitched at people who want a brilliantly-written singleplayer epic a la BG2 or IWD - it's pitched at people who a) want to create their own modules and worlds; and b) want to share those modules with other people; and c) people who want to play online with other people on a regular basis an a constantly changing environment.
Look at one of the reasons why the BG2 saga lasted as long as it did - it's because people began to develop 3rd party mods and editors (eg TeamBG, WeiDU, Shadowkeeper, etc) to allow people to customize their characters and create their own mods (because playing the game over and over again tends to get a bit dull after a while, especially if you play the game quite quickly). NWN will give you that option since they shipped their toolset with the game (shame they didn't ship instructions on how to learn how to script since that's what people really need to learn how to do to build great mods). I haven't even replayed the SinglePlayer campaign once since I finished it - I enjoy the unpredictability of playing with others in a persistent world (one where griefing isn't allowed, of course) and most importantly I've been busily working on my own mod - creating dialogue, scripting events, etc. THIS is what I bought NWN for - to CREATE, not just play the SP campaign.
I guess to summarize, people who are looking for a great storyline, excellent 2D graphics, and great gameplay without worrying about multiplay or the toolset will prefer BG2 over NWN - if you don't intend to build your own mod, play multiplayer online or download other people's mods, then NWN will hold very little appeal for you since the SP campaign isn't even a quarter as good as BG2 or even IWD1. Some people like going out for dinner, others prefer to cook their own meals - it all depends on what your palate prefers.
