I would go with PSP. you can load homebrew games and emulators w/o hacking it.
There is a program that runs on the PSP called "HalfByte Loader", it lets you load unsigned software on the PSP. Basically you start the HalfByte Loader and from that you choose which software you can run, it works 95% of the time.
Some emulators are already signed, so they can load them natively.
With PSP you can play MP3 and MP4 videos natively, not so with DS.
With the DS, it is easier to run DS ROMs with the R4 card or similar, then the PSP, and of course the DS ROMs are smaller.
In my opinion PSP is better for emulation.