I've heard of both being purchasable at as low as $129; so pricing it can be about the same. So tie.
Both have their own sets of hardcore users, those like me who enjoy older classic games will want the DS, whereas others who want more advanced features will want the psp. tie, so see next.
If you expect to do video and have more hackability, psp.
if you expect to have more features, psp.
If you want a large set of games now, the ds can read GBAdvance games as well as the ds games, so for games, DS.
It all depends what you want; myself i want both... so i can play my classic games (ds), yet have a pda-esque system (psp).