Of course the GBA is simpler to emulate. But on an official term, the N64 has more ability. Since that hasn't been emulated the company would do that one. If they get that emulation down, then the big hitter is under their control. The GBA would be more difficult in terms of firmware for the DSi. Since the DSi has an SD card, the firmware would have to be including some program to auto load the GBA to the SD Card, and portray that as the cartridge.

N64 is to Wii, and GBA is to DSi.

Nintendo will probably go with the Easier course of action and probably have it so that the game can run like they do with the Wii. Only problem with that is it's limited to the flash memory. They can also do the "Hey! This thing prints money." idea and sell some sort of app to play the games for X amount of points, and the Games for X'r amount of points. Similar to how an emulator works.

4 Months ago, this was an ok discussion. Now it is only dead... Who knows if the DSi is capable of such...