It should go without saying that a gutted Big XII is better than anything else BYU currently has access to - and is certainly better than independence! You could even make an argument that (with an avenue for access to a NY6 game) that G5 conferences are better than BYU's current situation.
Even if Texas and OU leave, BYU should absolutely join the Big XII if offered. Once BYU's foot is in the P5 door, it's going to be a lot easier to stay there - especially if the game of musical chairs is drawing to a close and the music is about to stop.