As far as raw numbers are concerned, BLM > SMN

BLM is also a bit more fun to level, with you getting your main AoE attack at level 18 instead of 30 or later like most classes. You also get into your main damage rotations a lot faster than SMN, who don't really change dramatically till they get Dreadwyrm Trance at 58.

SMN has the advantage of being a lot easier to do solo content with since they have their own personal tanking pet. The class is also pretty forgiving of input mistakes since their main damage comes from automatic DoT ticks and pet attacks instead of damage spells.