Resolve used to scale with HP and stamina early in the beta. It was a really bad idea, so that was changed. It now has nothing to do with either.
Resolve for a lvl 100 character is:
Code:
Resolve % = 100 * max(0,3.4*(1-e^(-.045*"Damage taken in the past 10 seconds"/36153))-1)
And if that makes no sense, just know it has nothing to do with damage reduction, or health, or anything on gear.
But the loss of the mastery is of course, a loss to %AP buff...
And a gain of %increased healing and damage, which are the same except they affect more heals. The 655 is pretty much always going to be better, because agi/armor > any secondaries anyway, and secondaries aren't far apart to begin with. Can't say that with 100% certainty because you didn't link the pieces, but I'd be very shocked if the 655 was not significantly better.