Like others I own both.
PSP : Big screen, great for movies, but does not easily play most formats like GP2X.
PSP : Perfect PS1 emulation (looks great on small screen) using Dark Alex firmware (he may be gone but his legacy lives on!)
PSP : GBA imho is better on PSP due to better upscaling of resolution, and also better compatibility (try Kingdon Hearts on GP2X, but it is a close call here).
GP2X : Overall, much better emulation quality on all systems apart from GBA and PS1. (controversial as some may prefer GP2X for GBA ... ). A lot of people say Amiga and SNES is much better on PSP. I think that is bollocks, if you can overclock to 250+ on GP2X, they are much better.
GP2X : Screen ratios - whilst a lot of PSP emulators include a mode to play in original format, you are only using a certain amount of the screen with black bordres - you feel dirty and cheated!

No probs on GP2X.
GP2X : You have the cradle, usb joypads, and TV out features to consider.
Basically both have pro's and cons, but are both great systems. If you are serious about emulation, but are not too fussed about PS1, I recommend the GP2X. I know you said you were already sold, but just adding a few pros and cons in case you have not ordered yet! Most emu's on GP2X are better than on PSP, just imo not the 2 i mentioned ...
With regards to homebrew, better selection on GP2X, but there are some good games on PSP too ...