I'd definitely go for cultist most of the time in a kingdom with mountebank and cultist in it. I always feel like mountebank is overrated, in general. Mountebank can only junk two cards in a turn and can be blocked. Cultist will empty the ruins pile ridiculously fast. Plus, draw > money. That's why mountebank also gives copper to make it balanced with witch.
(okay, mountebank is better than witch, but not by much)