While nobody can tell exactly what Victor will do, he's certainly worth taking a chance on. He has excelled at every level, just a matter of time I would say. It really depends on your league, but I'm guessing dropping Hernandez wouldn't hurt you much. Martinez may not immediately do much better, but his upside is definitely higher.
Catcher's not really a position I place alot of value on anyway, so to me it seems worth the gamble.