Here is a thread that might be of interest for you: PSP vsGP2x
I only have a DS so I don't have an 'enlightened' opinion on things
However I have been following the Pandora development from afar and I am really enthusiastic about it (waiting to see how the first batch works and then will probably be amongst the first to pre-order for the second batch) so I would tend to advise you to wait for it.
Notably I remember some people on their forums thinking that depending on how much cycles the Pandora can output for the dosbox port (and therefore emulates up to a 486 rahter than a 386) it might be a good solution to use the amiga version of the games of that era.
Add to that a full keyboard for text input and a touchscreen for mouse clicking emulation and you'll have a pretty decent emulation environment.
However, I suppose PSPUAE use a virtual keyboard (as does ScummVM DS) and use the analog stick to emulate the mouse.
So maybe in the end it depends on what games you play and how much in a hurry you are.
If you mainly enjoy A500 arcade-style games, you could probably go with the PSP.
If you want some AGA games, maybe use the workbench and some applications (if I remember correctly I read someone suggested the possible use of Final Writer/Copy through emulation as a word processor solution for the Pandora), it might be better to wait and see how things fare on the Pandora.
And well, if you get one of the first batch, please post comments here, I'll be very interested.