I think there is a lot of gut-feeling in this thread. The interaction between Masq/Sea Hag is pretty complex. I actually think this is something that is either simulated, or not understood (but thought - falsely - to be understood).
For example, Sea Hag/Masq vs Masq/Masq. Somethings to consider:
1) Sea Hag/Masq may give the other Masq player curses to give back to you, but that also means they didn't give you their estate that turn. This means that while your bottom line may be worsening (compared to if you didn't hag them), theirs is too (compared if you didn't hag them)
2) Masq/Masq collides MUCH more frequently than Hag/Masq for two reasons. First, the obvious one: Masq draws 2. So many times you have MCCCE, then Masq into another Masq. This won't happen with seahag unless you draw them both in your 5 (although Masq can still draw hag, but hag can't draw Masq). This is a small effect, but it's there nonetheless. Also, Masq/Masq will trash faster, and therefore will collide more as the game goes on. That being said:
3) Since Masq/Masq trashes faster, it can gain access to a trim deck faster. However, it has more to trash (curses as well as estates). If it can manage to get above the "hill", it can purchase a walled villiage (on this board), and probably stay "clean" from there on out. However, note:
4) Once Masq/Masq has cleaned it's deck, it may actually not be advantageous to play Masq anymore for two reasons... firstly, it may not have junk to pass anymore (this is on the assumptions that it has won phase 1), and second, it doesn't want to get a sea hag passed back. For example, imagine what happens if the Masq player passes a Masq (to get rid of the now useless Masq) and gets a sea hag returned. Now it has 1 less Masq to return the hag.
It's quite possible (and likely, considering the level of the players commenting in this thread) that Masq/Masq or Masq/Silver beats Hag/Masq, but I'm not entirely convinced based on these posts, simply because of the amount of assumptions that are being made, and all four of the above points which are not being quantified. Not only that, but I've seen this type of discussion literally hundreds of times before, and many many times it's completely wrong - even though there is a ton of groupthink that believes it is correct. This entire situation just sets my alarm bells off, because without a doubt, this board is SO COMPLEX in all the various possible permutations that there is simply no way on earth that someone can draw any accurate conclusion without simulating it. It's just way way way too complex. Sooo the entire point of my lengthy post (sorry)...
I think the only way to really understand this is to simulate it.
Tangent example (stop reading now if you are bored of reading my post
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It reminds me a LOT of what I read in MTG strategy boards. There will be two top tier decks and the entire community (including some of the best pros in the game) will believe that Deck A beats Deck B. However, in reality, after a major pro event, you actually find that all along the entire community was wrong, and it's actually Deck B that crushes Deck A. You then go back and read the reasons why the whole community thought the other way, and you find that it's all based on arguments that have been completely un-quantified, not simulated, and based entirely on gut-feel or small sample sizes.
TL:DR - I believe the case of Sea-Hag/Masq vs Masq/Masq or Silver/Masq is entirely too complex to fully understand without in-depth simulation. I may dig into this a little more if I have some time, because I'm not entirely convinced that the solution is presented here. This situation is just way too complex to understand without simulations or very very large sample sizes.
TL:DR#2 - I also think there is a second phenomena that is at play here too... A very very good player (say WanderingWinder for example) will post that he wins by playing strategy X. He then possibly concludes that strategy X is superior to strategy Y. However, especially in a game as little-understood as Dominion (newish game, small player base, especailly notably small community of theorists/simulations/stat analysis), there tends to be a large divide between player skills. The top players are literally three times as good as even the 2nd tier players. So WanderingWinder believes that strategy X is superior, however, he wins with it simply because he's miles better than his opponents, so he'd with with either strategy - executed perfectly. Which one gives him the bigger edge? It's hard to say.
This phenomena happens a TON in newer strategty game that aren't analyzed to *death*. I've seen it so many times. For example, the reigning world champion (and 2 time player of the year) in WoW TCG was adamant about a certain deck being miles above other decks. The entire community took the deck to the world championships (including him) and they literally all got CRUSHED. It was actually one of the worst decks out there. The world champion? He was top of the heap.
The effect he was seeing was not that his deck was the best, but rather, he was just really fking good at the game and could win with literally anything.
Again, not saying that's happening here, but it's a phenomena you have to be aware of.