Of course, we can't know the answer to this question without getting inside the heads of his teammates, but there have been some clues in after-practice player interviews.
The receivers, for the most part, have been generous in their praise of both quarterbacks. But several WRs have said that Bennett throws the ball hard, while Mariota has more finesse. Devon Blackmon in a video interview with Eugene's Register-Guard said that Mariota's ball "is easier to catch."
Several teammates have mentioned Mariota's athletic abilities. It feels as if there is more warmth heaped on him than on Bennett.
When you watch Mariota on video, you are struck by what a calm, easygoing manner he has. When I compare the two side by side, I would trust Mariota more in the heat of battle, and I think his teammates might do the same.
Without being in the locker room and seeing which of the two QBs commands more respect, however, this is all speculation on my part. And, while it's not a popularity contest, leadership is a quality that is very hard to define, but absolutely crucial in the QB role.