Mountebank gives out more junk. Two MB hits is the equivalent of being bit by cultist 4 times. 3 MB hits is the same as six cultist hits. Now mind you, copper is better than ruins and while cultist is fast and stacks, MB keeps giving. Both are very strong attacks. Cultist is a close second for me. However, I think playing an engine with cultist on the board is harder than when MB is present, bit I think this has now to do with players not buying enough MBs.
Minions real strength is in cycling. Ghost Ship is very weak and either very easy to counter or hard to play every turn, or just outclassed by the presence of another attack. However, when I started playing, it felt like the strongest attack in the game. Getting hit every turn was not fun. But, once you understand the card, it tends to become easy to play around.