I disagree with BlossomRose while Urthdigger nailed it on the head. Smns shine when a boss requires the party to move a lot or tank and spank while BMs are best with boss mechanics that require killing other stuff than just the boss. I would much rather have two BMs than two Summoners in a Sunken Temple of Qarn, but SMNers do really well on the last boss in Brayflox's Longstop.

So here is my suggestion. Level both classes to 15, then decide. We can't decide for you based on what the class does better. That is irrelevant because parties will take either 1 BM or 1 SMR or both in any party.