Tough one. For me, I'm only having one BM, so it's a bit different. But it was between my sis and my BFF.
For me, though, the question was what happens if it's 1 hour before the ceremony and I panic? BFF would slap me around, tell me to quit being stupid, get me a pina colada and tell me jokes. My sister would start a whole existential conversation about the nature of relationships and blah, blah, blah.
I opted for the BFF and the pina colada.