It has to do with the fact that at RAH, all FO's are paid the same. It doesn't matter which aircraft you start on, you are paid the same and seat/certificate locked until upgrade. At upgrade, you can go to any other certificate. You can continue to switch aircraft and certificates so long as it results in an increase in pay.

The only difference that mattered to me was the bases. The RW 170 has bases closer to where I want to live.