The GP2X is great, but Pandora will be way better (assuming its design / extra functionality appeals to you).
The main advantage of getting a GP2X at this stage is that it's got several years of software evolution behind it. The homebrew is great, most emus have been optimised for the system and run great (with new improved releases all the time), the devs are just starting to get the most of out of the touch screen, and the knowledge base in the community is vast.
Having said that, it's easy to fill up the GP2X with stuff right away but it can take a few months of experimenting, reading the forums and trawling the file archive to find all the stuff that's right for you.
Pandora has immense potential, and will likely come with a few good apps/emus out of the box. But it's going to take a bit of time before it has a software library anything like the GP2X.
From a portable media player viewpoint, the GP2X can be fussy and fiddly, and I have no doubt that the Pandora will kick it's blessed little bottom out of the box. Then there's everything else Pandora will offer - I won't list the immense feature set here, but you're virutally looking at a mini laptop. If you want a truly powerful, versatile device, it's Pandora all the way. If you only care about playing old games and want instant access to established emus, maybe a GP2X is the better option.
[edit] I should also add that I wasn't 100% happy with the d-pad on my GP2X until I modded it. I also had to add my own stylus holder. Pandora will not require such mods.
