PhatFarmer,
I don't know which route will "guarantee" you an SNA slot, but there are ups and downs to both. If you do NROTC, you won't have student loans when you graduate, but if you don't get SNA, you may be stuck with some URL designation you don't necessarily want. Another thing, since it seems like you may be interested in the Marines, have you thought about doing NROTC-Marine Corps option?
ElPensador,
If I were you, and I didn't get BDCP, I think I would just settle for a regular OCS slot. It's either 13-weeks after you graduate, or it sounds like your last year and a half of college will be a huge pain in the rear. I know us OCS guys only regret for not doing ROTC was that we didn't get our college paid for. Otherwise, we were happy to have college all to ourselves.