Try the original game first, and if you like it, pick up the expansion - that's what I did. I enjoy random levels, respawning levels, and the random loot. I still haven't put together a complete armor set yet, but you get bonuses for partials.
The expansion does have some nice features, but honestly, I'm not sure what features are just patches vs expansion only. My best recommendation is to go over to both
Soldak's Main Page as well as their
Forums to poke around, and check out the community.
I'm a fan of Indie Game Developers, and Soldak is up there. DC is my favorite of their games.
*edit* this is a snippet from their DC

W page:
"Din's Curse: Demon War is the first expansion for the unique action RPG Din's Curse. Adds the Demon Hunter class, new demon monsters, tons of new quests, more involved NPCs, lots of new dungeons and caves, and much more."