In terms of isolation...the beta 57, so if you have some super loud hi-hats close to the snare...the beta is definately the way to go. As far as sound goes, which ever one you like better. I'd try to just A/B them and see which one will give the result you're looking for. I generally run a 57 on top and the beta 57 on the bottom. I've gotten some good results that way. Since the mic is pretty nuetral (both of them actually), it depends a lot on your snares tuning.
fakeness,
Buy a SM57, as ong as you place the mic correctly on the snare, and your drummer can hit right, you should be as good as gold. You could buy a Beta, but you won't go wrong with a SM57. Save the money you'd spend on that beta and get a nice set of overheads or a killer kick mic.