The DS has some awesome homebrew capabilities, like wireless network/Internet capabilities. You can even use IM stuff like MSN, AIM and IRC. Oh and you can use your DS as a remote ... and a mouse for your computer. Very cool. You can also play SNES, NES, MS and GC/GBC games through emulation.
But screw homebrew -- we're after the actual games!
The DS has tons of awesome titles, with more coming up. The PSP has ... nothing.
Get a DS. Actually, get two -- just for the heck of it.
Sony PlayStation 2 (SCPH39004) v7 PAL
Matrix Infinity FW v1.82 - HDLoader 0.8b
Samsung SpinPoint P SP0802N - 74,5 GB
(Previously using HD Advance 2.1 Retail)