I'm not sure I'd suggest one for use just as a portable Mame machine. When I last tried mame on my PSP (a few months now at least) it was alright, but not great. Not worth the price of the system anyway. I'm not sure how fleshed out the GP2X emulation is for Mame, but it is cheaper.
If you want to run more systems, and are interested in at least a few PSP games, I'd get a PSP. NeoGeo/CPS1/CPS2 emulation is great on PSP, as is NES/SNES/Genesis... GBA is a strong emulator now. Lots of homebrew games are out as well.
If you want to do all of that, and some video, and want a few PSP games, I'd say it might be worth it (disregarding your financial situation for a minute). If you just want a portable Mame, I'm not so sure.