Quick answer: GP2X can run all homebrew/emus without the hassle of cracking the firmware.
Yes, Undiluted Platinum is around the corner (don't know? google is your friend), but the price can be near half of a PSP. And you'll need some technical people to install the modchip for you, if you don't know electronics/don't like open the PSP.
Also, GP2X can be refuelled on-the-go, as long as you find stores selling AA batteries - which the PSP couldn't.
On the bad side, GP2X can keep you waiting as the scene is somehow less active than the PSP one imho. otoh however, you'll find the wait is worthwhile when you see releases come out...