They specialize in different areas, so it really depends on the event and the rest of the party.

SMN's forte is their DoT focus, allowing them to continue to deal damage while focusing on other things. This makes them good on fights that require a lot of moving around, like primal fights.

BLM's forte is burst damage, as well as area damage. They are far better suited for taking out adds quickly, and are especially useful in primal fights to help take down the nails/plumes/heart that will make the primal's massive attack a 1-hit KO.

I'd probably say BLM, as when they're needed they're REALLY needed.