Some of the variables are pretty obvious, and you likely know about them. They are all cool features if you use them and like them. The wireless gaming and picto-chat capability with other players when you're nearby, the wifi connectivity supported by some DS titles, the use of touch screen and mic input for certain DS titles, as well as second screen providing other dimensions of game info or graphics for DS games.
Also you will exclude some titles from your choice by not selecting the DS.
Having said all of that, in my opinion there are limitations to what sort of games are enjoyable on such small screens, and for the most part GBA game titles offer pretty much of those choices that I think are suitable. Namely, Final Fantasy sorts of gaming, some old classic NES/SNES conversions like Castlevania titles, and a few relaxing things like golf and poker.
I don't care how great they make adventure and fps games look on it, I just don't find a point to playing them on either of these systems.
So basically, I love my DS, but the GBA port is doing a lot of the work