Faced with the choice I would go with Descent, but it does depend on why you want either of the games?
This. Two player games are a ton of fun.... for one thing, they're pretty much the only ones that eurogamers will tolerate player elimination. A good two player game is a battle of wills. Also, less downtime!
Having said all that, 3/4 of the time I'd rather go with an awesome group boardgame over an awesome 2player one. Don't get me wrong, the difference is minimal. It's like the difference between seeing a great movie with a friend or a group of friends. As I'm married and my wife is the person I can play games with easiest, I still do get a really good gaming fix from a good 2 player game like Agricola or Through the Ages.