Cool. Just keep in mind it's not 100% in the emulator (and the emulator has been put on hold/cancelled, so...)
Admittantly I'm biased, I have a GP32 but not a GBA. Wish I could afford both, actually.
Some people say that GBA emulation is not possible, others say that it's completely possible. Who knows.. time will tell. We just have to hope that the coders know how to exploit the GP32 to the max and exploit the similarities of their architectures. If the GP32 did GBA prefectly, well... it would be pretty much hands down GP32.. but at this point it's anybodies guess, so don't count on it in your decision.
I use the MP3 part of the GP32 daily on the bus, so it's invaluable to me just in that respect. Side note: if you do get GP32, get a cheap SM card reader/writer. I just picked one up for $13 USD ($20 CDN) and its the best investment. Instead of 45 minutes (at least) for a full card of MP3's, it's only about 6 minutes. Much more practical.
Good luck with your decision. Remember, there are always GBA emulators to get a feel for the *cough cough* public domain roms out there too. Do your research and check out all the info you can, and make your decision on that!
Well, time for me to go and brush up on my programming. 
