The best system for portable emulation is the psp hands down. 3.52 m33-4 cfw is amazing. N64 emulation, PSone emulation, perfect gba, neo geo, cps2, cps1 arcade emulation.
Mame emulation is better on the gp2x. Also, the genesis emulator on the psp is 99% real speed of games on the psp, so...