If they both upgrade into PN, then B is always going to be cheaper than Y. And booking in F is usually cheaper. E.g., EWR-SFO (a few weeks ahead, so all buckets available) is $1625 in Y, $1424 in refundable P, $1027-1077 in refundable B, $795-845 in refundable M. Of these options for getting a F seat, Y doesn't look like the winner. There may be very rare times when booking in Y to get an F seat is the best option, but most of the time it's an inferior option whose sole advantage is that it 'looks like' economy to the employer or reimburser.