That depends on which your going to ride more. The Anthem which the Flag is more or less the wide version of from what people have said will kill everything, park, freeride, pipe whatever. The Slash on the otherhand is leaning more towards the freestyle/jib aspect. If you want to freeride and are looking into riding kickers and bigger features then the Flag should go good. If your looking to jib around, well buy a cheap wreck board to jib with!
But if your gonna ride alot of park then probably the Slash, but if you want to bomb the mountain and also be able to shoot through the park features then the Flag wont go far wrong.