Guy with the most experience, go with the senior and the QB with the more athletic ability.
Gives more leadership and gives the O another dimension the D has to worry about and pre pare for every game. If the D must have a defender or defenders watching the QB in case he runs it takes that defender away from covering a receiver or running back, so opens things up more for the offense.
The team does not gain the same benefit if Gerry comes in as a back up or only on occasion.
Retzlaff has another year and will still get to play a lot as the back up this year and then be ready to lead the team as the number one guy next year.