PSP with downgradeable firmware is best.

A modified psp is the ultimate portable accessoire. You can play _any_ PS1 game on it, there are emulators for GBA, SNES and almost every system before them, N64 and NDS emulators are also available, although they don't work perfectly yet. Watching ripped dvd movies, listening to mp3, all no problem. And then there's the homebrew scene... if you wanted, you could even run linux or Win95 on the psp, use it as a television remote control, install a ogg / divx player, change the menu icons and the startup videos - the possibilities and the amount of available homebrew is endless.

And about the battery - in the new m33 firmware, there's a built-in tool to lower the cpu frequency of the psp. 100 MHz (standard: 222) is enough for almost everything except gaming. With that, the battery runtime problem isn't as big as it was before the custom firmware.


But apart from all that - if you don't want to modify your psp, then the NDS is a lot better. The psp hardware is great, but with Sony's firmware restrictions it's just crap.