Key is you're asking about casual, unaffiliated fans - not fans of BYU (or the opponent).

So I think the VAST vast majority of unaffiliated fans who would watch BYU vs LSU would also watch Utah vs LSU (assuming same time, channel, etc).

The one difference would be if either team had a player that was in the national conversation (see Detmer as a Heisman contender, Alex Smith as a likely high first round QB, or - for a more recent example - Max Duggan as a Heisman finalist). Then that would be enough to give that team an advantage.

If those factors were also equal, then a very slight edge to BYU, just because BYU is inherently unique and stands out a bit more than another University of [State] or [State] Tech/State