I own both, (in b4 IDORT)
I'd say DS.
PSP is a great emulator machine, but once you go through the library of maybe five to ten (metal gear anything, gta anything, locoroco, patapon, and so on) truly awesome games, you'll quickly grow bored with it. But it's awesome because of the GBA, neogeo and snes emulators which work near-perfectly.
The DS just has so many games, it's a geekgasm. Plus, I much prefer the homebrew on DS, with of course a slight advantage in emulators to the PSP.
Get a DS and a PSP, both fat and already open to custom firmware lovin' on ebay, for the price of one psp or ds lite, is what I say. Both are very complimentary.
But seriously, DS.