I've never played either Dungeon Keeper game before, but am curious to try one. Which one should I get?

It depends what you're looking for in the game.

There's been a healthy exchange of opinions in this thread: http://www.gog.com/en/forum/dungeon_keeper_series/dungeon_keeper_1_or_2

I don't think either game was a clear winner. However it was generally agreed that the second one has a better interface, and is more playable, but the first one had better missions and gameplay, but has aged quite poorly in terms of graphics and interface.