If the PAC12 and Big12 both made offers tomorrow, I'd say go to the PAC12. I think BYU is a better fit there and the conference is more stable.
If BYU were a Big 12 member today and the PAC12 came calling, I would not make the move unless I really feared for the stability of the conference. After 3 years, we would get a pretty good idea of how the conference is working or not working, and I would prefer to stay than bolt like Maryland did to the Big 10.