Bid the 50, fly for a year or two, then bid the 70. You'll hold a line quicker and have better QOL. You don't want to be on reserve if they are short on FO's. They have even been offering 150% pay for trip pick-ups on the 70 because of the shortage.

Plus any SLC flying that may be done soon will be the 50. It will be nice to get a change of scenery.