Utah plays a 4-3, where the linemen pin their ears back and come at the QB hard. The LBs often play close to the line of scrimmage, and the DBs play bump-and-run coverage. It's a high risk, high reward defense that is fun to watch.
BYU plays a very conservative 3-4; the down linemen pretty much just take up space, the LBs are the main tacklers, and the DBs play 7 yards off the ball, trying to prevent the big play. It's a boring, but effective defense.
For coach Sitake to come to BYU as the DC, he would almost certainly need to adapt to Bronco's philosophy - which would be a fundamental paradigm shift for Sitake. Considering his high level of success with his current system, I don't think that a BYU DC job is likely. And he deserves better. I think the only way he goes DC somewhere else is at a top-10 school (Oregon, Alabama, Michigan State, etc). Anywhere else, he's head coach salary and head coach material