On most aoe packs, MP is never something you'd run out of, so not taking advantage of the MP reduction on Ruin 3 isn't a loss.
Then on basic potency (between brackets, includes DWT buff): Blizzard 2 = 50 (55) potency per target, Ruin 3 is 200 (220) potency, this means B2 is a better choice in terms of raw potency at 5 targets or more, and equal at 4.

Early deathflare is a thing as well, you want the deathflare to actually hit when most of those mobs are still alive. If 1 mob dies before you use it, your deathflare does 440 less potency (always buffed by DWT after all). Using Ruin 3/blizzard 2 inside DWT instead of outside, doesn't weigh up against that potency loss.