Whats the difference? Which to buy?

If you mean Neverwinter Nights Enhanced Edition and Neverwinter Nights 2 Complete, they're two different games. Both have their own fanbase, so some will tell you to get the first, some to get the second. I'd say if you have the money and are interested in D&D RPGs, get both.

But, note that:

- Neverwinter Nights 2 Complete (created by Obsidian) contains the main game, both expansions, and the one Premium module made for it. It's really complete.

- Neverwinter Nights Enhanced Edition (originally created by Bioware, "enhanced" by Beamdog) contains the main game, both expansions, and a couple of short Premium modules. You also get Neverwinter Nights Diamond, the last edition of the original game before Beamdog's Enhanced Edition, also with both expansions and the same Premium modules collection. It's a bit controversial whether and to what extent Beamdog actually enhanced the game, so that's great: you get both versions and can decide which one to use, or even both. But there are several Premium modules / DLC that are not included and that you'd have to buy extra: Pirates of the Sword Coast, Wyvern Crown of Cormyr, Infinite Dungeons, Darkness Over Daggerford, Tyrants of the Moonsea, Dark Dreams of Furiae (the last one has bad reviews though).

- The idea behind both games was to give players a tool to create their own D&D campaigns and adventures, so apart from all of the above, there are hundreds of free modules (= user-created stories/adventures, independent of the main campaigns) that you can download, and a lot of them are just as good or better than the official content, so unless you're a collector and/or completionist, you don't really need to buy all of the DLC, or not yet anyway. You'll get several hundreds of hours of gameplay even without them.