Oen. Whichever fleet has the better lifestyle now, means nothing for the future. Things change regularly here. I believe there are 93 380s already here. Press states a final fleet at 120. So commands for anyone who has not been on property for a while already are a long way off.
The 777 fleet growth is also stagnating. With most new deliveries being replacement aircraft for older ones. So again, those joining now, or not already here for a while will be waiting a long time to change seats.
As gardenshed has stated, the choice of Fleet offered is not yours. Both are great aeroplanes, and the rostering system seems to be able to ignore bids from both fleets equally.