thought I'd stick my oar into this pointless debate
most of the hardware quality features vs price thing balances out pretty well between the two.
it all comes down to whether you want to play:
a. wipeout and gta, etc. - get a psp
or...
b. noiz2sa and gauntlet (and nebulus and...) - get a gp2x
plus there's the issue of open platform or not. you've got the choice of:
a. kick-ass commercial games (at commercial prices) with super-whizzy graphics - get a psp
b. lots of freeware (lower quality and more hassle to setup, but it's free!), plus the ability for you to easily write your own code - get a gp2x.
personally I don't understand people who have a 'brand loyalty' thing going on. I'd love to have both (I DO like wipeout!), but can't really justify the cost
so I'll just stick with my shiny gp2x as I'd rather take the b. options above