I'll go out on a limb and say Joshua by stoppage. A.J. simply fought the wrong way against Usyk in the first fight, letting him set the pace of the action and not using his size and power advantage to his favor.
He lost to Ruiz, came back and embarrassed the guy in the second fight, but it didn't help Ruiz to come into that fight even fatter than he was in their first bout. Usyk won't be that guy and will come into the ring in great shape but I feel if Joshua wants to force the action, it will eventually wear down and overwhelm Usyk.
Do I care who wins-------not really. Just going with my gut feeling.