The GP2X and its predecessors are great handhelds for retro emulation fans. It definitely has a wide range of emulators and other software already available due to the easy recompiling of existing Linux code for the platform.
And this new one has a faster 533MHz Arm9 CPU so emulation should run well.
I don't know anything about the other one though. At $100 it isn't really expensive. About the same as a mid range MP3 player, and it it does have battery life was claimed and it well made with good controls and screen then it could be worth it.
However I have a PSP and run a lot of emulators on that and it does me for the moment. SNES games run especially well.