I think it all depends on the evaluation of the current QB draft class.
If Idzik thinks there's a guy out there who is the long term solution at QB I think he'll be aggressive at getting him with draft day trades (we know Sea was not afraid to deal).
In that case it will be Sanchez, Rookie and McElroy with the rookie getting the nod if he shows any semblance of progress in camp.
Remember Rex is coaching for his job and of he starts a rookie who sucks at first but shows progress at the end...he'll have better chance at saving himself. If he starts Sanchez and the rookie doesn't play until halfway through the season Rex will get slaughtered for not giving the rookie a chance to develop.