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.
As much as I want AJ to win, I think ultimately Usyk is too good technically and will ride out some hairy early attacks to win on points. I wouldn't be surprised if AJ wins by KO in the first 4 but it's definitely the lower odds outcome I give AJ maybe 30% chance of managing that but otherwise the Ukrainian will retire the man from Watford.