I think the case being made is that, nationally, BYU will earn more credit/clout for having such a win. The common CFB fan will recognize BYU for that win more and longer than a Utah one. BYU will soar up the ranking higher as well.

For the beat Utah side, I think the reasoning is more practical, in analyzing the actual long term benefits of beating Utah. The case being BYU will win more/better local recruits and lay a strong foundation for continued local success which will also create a national foundation for success...

I personally think both have merit, but it just depends what matters most to you...