SEA HAG/SILVER: would reasonably expect to play Sea Hag on t3 or t4, and buy more silvers on both t3 and t4. (well, that might be slightly optimistic, but let's just use that as the baseline) Hag could miss the reshuffle and turn up on t5; chances are 2/12 = 16%.
End of t4: 7 copper, 3 estate, 3 silvers, 1 Sea Hag.
SILVER/SILVER INTO MOUNTEBANK: would reasonably expect to buy Mountebank on one of t3 or t4, and buy another silver on the other turn. According to the old front page post, you have an 8.8% of not reaching $5 on t3 and t4. As P1, an opponent's Sea Hag would not change these probabilities. As P2, your chances of not reaching $5 are worse if the opponent plays Sea Hag on their t3.
End of t4: 7 copper, 3 estate, 3 silvers, 1 Mountebank, 1 curse from opponent's Sea Hag
Both strategies give you one curser and one "dead card" (if you count Sea Hag as a dead card, which it kind of is in terms of your own deck's production). The Mountebank deck has slightly better economy because the Mountebank acts as an extra silver. The added copper junking hurts some, but the moat effect is also not so great for Mountebank.
Overall, not as much difference as you might first imagine between the two, as the simulator shows.
I would add that to DG's post and suggest that the more important questions might lie in the endgame. What are good duchy/estate/even province buying rules when you have 5 curses in your deck? We know the rules of thumb for standard BM games (buy duchies with 5 provinces left, maybe take a gold with $8 if it's real early but otherwise go straight for provinces) but I don't have a feel for how these rules change with lots of dead cards.
Should you build for longer and add more golds/silvers to your deck before springing for provinces? It seems reasonable but your cycling would also be slower because of the bloatedness of your deck. When should you buy duchies if you find yourself with $6? What about with $5?
