Both DS and PSP are locked down consoles that have to be hacked to dev on it. If you're the author of a free (or open source) emulator, you would tend to gravitate towards an open platform. There's the PC of course, but in handlheld consoles you have the GP2X. It supports a version of SDL so porting is apparently extremely easy (as long as you can fit stuff in a smaller screen).
You got to know what you want it for though. I went for a GP2X because I found myself using my GBA only for emulation... I would only play a regular game occasionally. PSP has a bigger screen and much much better 3D, but personally I don't care much about PSX emulation for the moment. There might be a next gen open portable console that can handle it. In the meanwhile I get 60FPS CPS1/2 and Neogeo on the commute (without sound though - it slows down a bit with it), 6-7 hour battery life, and a very good 320x240 screen that doesn't ghost.
Open consoles are rare and slightly more expensive because the company needs to make money on the unit itself, not on the licenses. But it's a great experience, you just put crap on your SD card and you're ready to go. No dicking around with expensive flash carts or weird software. It plays DivX so you can also use it as a media player. There's even a port of mplayer for it so you don't need to convert your movies if size is not an issue
The only downside is that it doesn't have a keyboard (they added a touchscreen recently to the latest GP2X, but not the same). Here the DS could be better as you'd use the 2nd screen for input. But truth be told, I rather emulate a PC on a small computer - like the EEE PC. Closer to the real thing, and still very portable.
With those two you're pretty much set up for anything that came before the PSX.
It's quite a large back catalog :-D
[download a life]