this is just my opinion but gba games usually sucks on a pc. The game looks ugly when its set on fullscreen and becomes messy when you minimize with all the icons around. playing through a keyboard is also pretty uncomfortable and while you can hook up a controller, it feels weird using an analogue stick and the dpad isnt that great. And you cant fit a pc in your bag and take it around and play it on public transport. plus it's illegal. :\
im going with ds but not because of the gba slot, i use my gba sp for that, but because of the region lock. While it's only pokemon bw that i care about, it's a big thing considering its one of the greatest games i have played in a long time.