If you're looking to play GBA games on the DSi, there's three choices, but only one really worth making.
1st is the iPlayer, originally designed as a homebrew only flashcart with a dedicated CPU, Darkchen was allowed to make a GBA emulator for the flashcart, which worked surprisingly well.
It cannot play commercial DS roms.
2nd is the EX4i, another R4 clone but with an integrated CPU as well. It's GBA emulator however is very basic, lacking any configuration or advanced features.
3rd is the Supercard DStwo, which has an updated version of the iPlayer's GBA emulator, with fixes and improvements that make it the best GBA emulator. Even without the GBA emu, DStwo stands to be the best flashcart out of the three listed.
So there you go, pretty much settle for the DStwo and nothing else.