BYU had a very "interesting" decade in the MWC. Crowton then Bronco. Under Bronco, BYU competed for a conference title every year but his first. So which part are you comparing to?
Utah was the last team in the newly formed P12 to appear in the CCG, but, to their credit, haven't looked back since. Still have yet to actually win a title.

BYU was the biggest fish in the pond in terms of resources, fan support, and commitment.
Utah is middle of the Pac.

I think this board (and fans in general) try too hard to compare the two programs. Both have had a completely different path and I suspect will be very different going forward as well. I don't buy into the demise of Utah football like most CBers do. They will continue to do just fine as long as they have Whitt. Afterwards is a crapshoot, as many P5 schools are learning very hard lessons about the last few years. There's no guarantees when hiring a HC.

I also think BYU will compete in most games in the B12 immediately, but I don't see them competing for a conference title right away. I don't recall exactly but feel like Utah was involved in a lot of blowouts (on both sides) early on in the Pac12. They were kind of up and down, hot and cold (I'd rely on the Ute here to verify my recollection; I don't really follow Utah football). I feel like BYU will be lukewarm for much of the season: they'll keep every game close regardless of their W-L record (which I suspect will be around .500).