I think SMN has the edge in A3S, if and only if you can get past the Pain and tornado phase without throttling the raid's CDs specifically because you have a SMN (in our gear level that's hard right now, next reset when we all have weapons, I think not). It might be because I'm not playing it right but when I go SMN, we fail the Pain check because we don't save stuff for it, but with BLM the burst is real enough we can get through it 90% of the time. I see no point to BLM over SMN once you're in final phase, it's really quite a lot of moving/mechanics that must be obeyed immediately, and current strats have caster LB3 twice in that phase, which clearly favors SMN.