- If there is a domestic leg your Y will get you upgraded on booking and definitely top of the list if it doesn't get you there
- Your more expensive Y will get you more MQD and skymiles

- Higher priority in case of issues/overbooking (though DM status puts you high up there anyways)
- B may sell out if you're looking to do SDC; Y will be less likely
Strategy of booking Y/B and switching to Z sounds like a winner if you can.