If flights are very full, Y should have better availability than B. Both earn 150% MQMs, but Y is much higher in the hierarchy for free elite upgrades on domestic flights and IIRC *might* require fewer miles to upgrade. Y might make OPUPs slightly more likely. Y should also be easier for SDC, although if both are changeable without fees, there probably would be no reason to do a SDC at T-24 rather than simply changing the ticket in advance to the new flight.

However, it sounds like the OP just wants to park funds until travel plans are firmed up and then change to the ticket the OP really wants to fly.