Right now I'd say DS. It seems to have the most support of the two, plus there are a good number of games that put it's unique features to good use. Plus, there's the high-caliber first-party games from Nintendo, which are a major bonus if you like them.

However, definitely keep your eye on the PSP. This coming year sounds like it'll be the one that PSP fans have been waiting for, and honestly, though I don't own one or plan on buying one soon, some of the games coming out for it have me green with envy (notably Castlevania: The Dracula X Chronicles and Final Fantasy VII: Crisis Core.)