try logging in with one and go to your service record. if that one doesnt work use the other one. you have to click a button so there is no way that it would not unlock it if you try one way or the other.
I would assume your windows live one is the one to use. I didnt log in with my bungie login (which is different than my XBL email).
*Kudos to dragonpwnr99 for the Saber Sig