In terms of witch vs mountebank, I think it depends primarily on trashing. If there is no trashing, witch is stronger because mountebank will stall out, and because without trashing in a curse game, you're probably going to want to basically go for big money.
For young with vs sea hag, I think unless the bane card is good, it's better to go with young witch. In a curse game, the discard 2 doesn't matter all that much, so it's basically a witch. Sea hag has a stronger attack, but it's semi-countered by young witch. Having the curse on top makes one of the cards in your current or next hand useless, but since the young witch turns look at 6 cards (besides the young witch), the hit is not as bad. The young witch player still gets to choose 4 out of 5 cards to use (the 6th being a curse), while the sea hag player just has to play with the 4 she drew.