Step 1. Level both
Step 2. decide which you enjoy more
Step 3. Decide whether playing what you enjoy more is worth the possible difference in DPS.

Fact is though BLM are doing splendid in damage numbers HOWEVER they require a lot of forethought to a fight and maximizing every single little ability you cast and if you don't know what's going on at every second of a fight (at least with regard to EX primals) you're gonna have a bad time. SMN are far more lenient in this regard, however I can say without a doubt having played with SMNs of similar skill level, in Bismark a SMN will beat you if played right. In Ravana, you will be a SMN if played correctly. Fights favor different jobs just the way things are. Just so happens BLM require a lot of background knowledge and familiarity with a fight to be as effective, but once you are familiar with the fight? You can expect you could even beat a few melees out there, especially with a foes req at your back during burst phases.