In addition to what you wrote, I think a big factor is where BB thinks the team as a whole should be going on offense, more towards a running approach which favors Cam, or a passing approach that favors Mac.
Curran and Perry said that even in training camp the play mix gets weighed more towards running while Cam is in and more towards passing when Mac is in.
Personally I'm not a fan of the shift towards running QBs in the league, and gotta say I was disappointed with how the team looked with Cam last season. Things were OK when we stayed on schedule with down and distance, but when we got third and long it was ugly. I feel we will have a better chance with Mac, once the coaching staff feels he is ready to step in.