Koby is right on this one, a lot of Rasmus' value does lie in the fact he could be a GG-caliber CF in the Bigs, which is fairly rare. But with an upside of .300+-25-30, I think he may be placing too little value of his offensive abilities and too much on his defense.
However, Snider is like Butler was a couple of years ago - no position, awesome bat. In fact, comps to Butler are not unwarranted - Snider's bat is fairly polished for a HS hitter, he has excellent power, and is a very good overall hitter with a good approach at the plate. He strikes out quite a bit but I'd assume his K-rate will go down as he gets more experience. Snider's ceiling is a top 8 prospect with top 3 hitting skills. I don't think a ceiling of .320-40 is out of the question, though that may be very generous for a kid who's just going to be 19 going into his full season debut.
Tough call, but I'd stick with Rasmus on this one because of the fact that regardless of whether his XBH develops to hit 20+ homers, he's almost certain to make the Bigs just because of his defensive ability. He will be able to steal bases in the Bigs, and he is a good hitter who will hit, at the least, .270. So his floor would be better than Snider's.
OF (x3): CarGo, M. Bourn, D. Jennings
UTIL (x2): E. Encarnacion, C. Hart
BN: Cuddyer, C. Ross, J. Montero
SP: Price, Gallardo, T. Hudson, Lester
RP: Chapman, Jansen, Rodney, Putz, Cishek, Bailey