I've done emulation and homebrew with both. Personal preference was for the PSP: Much better SNES/MegaDrive emulation, C64 emulation is pretty good, and PS1 emulation is near flawless.
I'd agree that there's a bit more homebrew for the DS, but the PSP is a beast for emulation.
Since this is an Amiga site and all, I'd be remiss if I didn't mention FOL's excellent PSPUAE. Check it out: http://www.pspuae.com/