I have a GP2X and a Psp which is 1.5 firmware. I do not really use my GP2X much at all but I use my Psp for all emulation which includes Snes, Nes, Sega, GBC/GB, C64, GBA and more. It is possible to run emulators and homebrew on all firmwares at the moment but 1.5 is the best for ease of use, convenience as well as full kernel mode access with 2.0 coming in at a close second.
As time has gone by, more of the Psp's hardware and capabilities is being utilised in emulation, the most recent being the Media Engine and before this being it's 3D GPU.
The Psp has a standard 333MHZ CPU, a second CPU called the Media Engine which can run at 333MHZ, a third CPU (or 3D GPU) which can run at 166MHZ and it also has VFPU.
The next release of the Snes emulator will be utilising all of these but what will make the most impact (in my opinion) is that the entire sound emulation will be moved to the Media Engine, the 3D GPU can already be used to deal with the graphics emulation so this will leave the main CPU with less load. There will also be wifi which has already been demonstrated. The snes emulation is near full speed already so I expect even more when all of the sound emulation is being dealt with by a totally seperate processor.
When it comes to the GP2X, I think it is already very good but it just needs time to mature. The Psp has been out for over a year so of course emulation will be at a higher level, however I do think that it will most likely stay a step ahead due to its power and 3D capabilities. With regards to the Psp's wide screen being a disadvantage, I have not found this to be so. The most recent Snes emulator has screen caliberation which allows you to adjust the screen exactly how you wish.
My advice would be to purchase any Psp that has firmware only up to 2.0 because this can be downgraded for 100% homebrew or kept as it is for 80-85% with no kernel mode.