Ive had my psp for about a month now and can't get enough homebrew. I love the gp2x, but I don't think it is has fulfilled my expectations just yet. I beleive that in about a year, there is a good chance that I will be using my gp2x way more than my psp, but for now - that is not the case.
I love the screen of the psp. I know how emulation is fits better on the gp2x's, but when I watch a movie on the psp, its definitely worth the black bars on the sides. I don't mind them at all.
As for emulation, the gp2x is great at what it does. Drmd could kick ass against any genesis/sms/gg emu for psp. However, I think it is worth the trade-off when you can get a great snes emulator and the genesis/sms/gg emulator for psp is ok. Mame wasn't something I was really psyched about having so that wasn't really an issue. I find it hard to get roms working on both.
The wifi capabilities of the psp open up so much. I can't tell you how much I love holding a portable web browser, email reader, and vnc client in my hand. The battery is very different. For me, plugging it into the wall is much easier than pulling off the back of my gp2x, getting the batteries out, and putting them into the charger. I have already lost the back to my gp2x battery thing.
If you sold your psp and bought a gp2x, it would definitely not be a mistake, but you might consider waiting a bit. By the way, I still have and play my gp2x. I love the psx emulator that just came out. Thanks zottd. These are the types of things that will help put the gp2x over the psp. Its just a mattter of time. Im lucky that I have both.