This decision is complicated by the fact that he has already used his redshirt season. If he transfers now, then he could play next season where ever he ends up. if he stays and plays this year here, then transfers, unless he can finish his degree by the end of this year, he will have to sit out a year, then only have one year of eligibility remaining.
Now, there is also the very real possibility that he will be needed this season much like last season. In addition, Mariota could struggle early and lose all of his confidence in which case, Bennett may ascend to the starter and put a strong-hold on the position. Does he risk this, though, and stay, only to have Mariota stay healthy then Bryan only gets mop-up minutes?
From a playing perspective only, it makes more sense to transfer now and have two years of eligibility left. From the "what's right" perspective, he committed to playing at the University of Oregon. Kelly did not promise him anything other than an opportunity. To leave the team this late in the process when the next two in line for playing time are true freshman may not be the right thing to do as a team-mate.
Let me just say that I am glad I am not in his shoes.