Have had awful experience with both cores.
mGBA is more stable but it's having sound issues and occasional "hiccups' (not sure if frame skipping?) with certain games. The latest version to work fine, for me, is the core from RetroArch 1.9.1
Beyond that I'd say, stay away.
gpSP on the other hand, I've found to have compatibility issues, some games won't load, others will play fine only to crash the emulator a few seconds later, and some roms won't even load only to crash the emulator. I've avoided gpSP since RetroArch implemented it.
Also, what platform do you plan to use RetroArch with?
Android? PC? Console?
RetroArch doesn't work the same in every available platform. I've had issues with Wii RetroArch... but everything works fine in Android and PC.
Also, when you download separate cores they comes with .info files.
Opening them with a text editor (Notepad) will list all it's features. Might be worth reading said files if you need that kind of information.