Depends. Do you want
cheats and/or save states and/or the ability to hibernate? Use PSP.
GBA VCs don't support VC's snapshots nor the ability to close the system. Plus while you're able to cheat I believe it involves applying the
cheats to the rom (before injection) or to the save. Either way you have to have your pc to turn
cheats on.
That said, GBSP is more likely to drop framerate, crash, corrupt a save, etc. while if playing as Nintendo intended the VCs run like a dream.
You just have to pick gameplay or tools and run with that option