The direction to go will depend a bit on your taste in games- both systems have upcoming titles that should appeal to you.
Having had both the PSP and the DS, however, I found myself making more use of the PSP. At the time I had the DS there wasn't much there that really appealed to me other than the Mario Kart game and the Castlevania games (in particular Aria of Sorrow). Meantime, on the PSP front, i was playing Hot Shots Golf: Open Tee, Bomberman, Daxter, Medieval: Resurrected, etc.- i just found more there that appealed to me. This, along with the other abilities of the PSP, give it the edge in my book.
So, if it were me, i would be going for the new PSP over the DS Lite.