The DM before me is the odd-man-out; he always chooses Foundry. You'd have to ask him exactly why, though--every time I ask him what he likes about it, his response is something like "because it isn't Roll20" and while that's true, it's not a useful answer. He likes to stream animated maps, animated tokens, sound effects, ambient music, and animated spell effects, and Roll20 can really struggle with that stuff on nights when all their servers are busy. Foundry runs off of his computer, not a distant server...so he has a little more control over bandwidth. Maybe that's the biggest draw for him? It certainly would be for me.
That's the biggest reason I'd change to it if Maptool wasn't doing it; not because of bandwidth (I've got plenty of that these days) but because I'm not dependent on a third party's server.