Its easier to use than a DS and pokemon only really needs the buttons it already has, the touch pad and all that just add more complications to the game and constantly reminds me how useful a third arm would be.
I believe the GBA would be a better system to use because, not only does it not have all that extra stuff that Shibby said earlier, the game was intended to be used on this system. I laugh and mock, and sometimes even slap the people who tell me that they like to play games on emulators on their computers because they call themselves gamers, yet they don't even respect the games that these so called "gamers" play.
So out of the respect for one of the, in my personal opinion, best portable systems in the world, I believe that it should be played on the intended system.
Pokemon Emerald would be better played on the Gameboy because Emerald is the best game out of all the Pokemon games, and the Gameboy seems to fit its personality. It's a small and easy portable gaming system, and you can pull it out wherever and play Pokemon Emerald, but the DS is too big and bulky, and for a game like Pokemon Emerald, I don't think it is advanced enough. There would be no use for the touch screen at all since Emerald has no use for a touch screen. So then the bottom use of the DS would just be a waste, so might as well stick to the Gameboy, where Emerald uses the system's full potential.