I think it'd be a better idea to compare the two after BYU has played Cal
Using ESPN's offensive efficiency ratings, if you pool BYU/Utah's opponents, Cal will be the 2nd best offense either team has played (Oregon obviously #1):
Even then, BYU's 2nd hardest defensive test is way down at #42 (Boise State), and Utah will have played SIX teams better than that, so it's still not a very fair comparison.