AFAIK SS nullifies all dmg la
because i know all the lazy DHs that havent switched to gloom (or run both SS and gloom) just SS on RD damage lol.
so ALL IN ALL SS > serenity by a long shot. (more so if you still run legacy nat for that stupid disc regen

)
anyways for resolve, its pretty nice. belial in mp5/6 deals about 30k+ per slam with me having ~550 AR and about 5.8k armor
resolve basically -20% that (or his actual dmg -20% before taking account of my DR, can't be bothered on the actual math so help me dig that one out la XD) and he deals ~15-20k instead.
i got 28/29k hp.
TBH, if your gear is great id reckon you're better off with something that you can scale better with. resolve will never scale as it is scaling based off monster's raw damage. (unless you mean mp2 deals more dmg than mp1, and -20% = more dmg reduced = scaling, lol)
AFAIK it helps on AOE too though i havent got the time to go find some imps to verify lol. too busy getting angry at KWs never dropping keys lol. (when i get to play anyways)
as for belial, it's pretty clear cut he's alone and it definitely works XD
Yaka? I thought SS deosnt nullify molten/archane/plague? Cause last time I use DH on SS run through those, life still drop.