I own GP2X and have a PSP
Brief comparision:
NES:GP2X wins, PSP screen is too ghosty and filter blurry.
SNES: GP2X wins, PSP screen is too ghosty and filter blurry..
SMD/GENESIS: GP2X wins, PSP screen is too ghosty and filter blurry.
MAME: GP2X wins, PSP screen is too ghosty and filter blurry, bad emulation.
CPS1: GP2X will win when an emu is released, PSP screen is too ghosty and filter blurry.
CPS2: GP2X wins, PSP screen is too ghosty and filter blurry.
NEO-GEO GP2X wins, PSP screen is too ghosty and filter blurry.
C64 GP2X wins, PSP screen is too ghosty and filter blurry.
AMIGA GP2X wins, PSP screen is too ghosty and filter blurry.
PSX - PSP wins, even though PSP screen is too ghosty and filter blurry.
PC-Engine GP2X wins even with bad sound, PSP screen is too ghosty and filter blurry.
Total:
GP2X - 10 wins, PSP - 1 wins