Joshua.
Wilder has to take him out with a fight ending shot very early in the fight or I think he's brown bread. His power is clearly ridiculous, but he usually lets guys get into the fight, and lesser fighters than Joshua have been able to soak up his power past the halfway point.
I'd agree that if Wilder got Joshua hurt like Wlad did in the 6th he'd probably finish him, but I don't see Wilder lasting anymore than 2/3 rounds, his window of opportunity is small. The problem he's got is not necessarily getting blasted out with one punch, but when he gets hurt he doesn't have smart defensive instincts or the physicality to tie up close, and that's going to be terrible if Joshua starts unloading combinations on him. I don't think he will be able to weather the storm like Klitschko and those stick legs are going to have to pay sometime, we haven't really seen hit by anyone his own size yet.
At boxing distance it's an even fight, Wilder has the quicker jab but he's inconsistent with it. Joshua has a good jab that he uses to back guys up with too, whereas Wilder's is more of a guard opener for his right hand. I think both guys can land those long right hands. At this distance though, Wilder has the better shot of a KO as his straight right is more devastating at that range, but when AJ gets up close, it's going to be difficult for Wilder do anything but take punishment.
If I had to bet how it'd play out. I imagine both fighters start out boxing at distance but by the 2nd round it's already heating up. Wilder will throw the right hand, probably hitting a bit of glove and not landing flush, but enough snap to look hurtful. AJ works the body and sets up a combination that backs Wilder up to the ropes and then drops him with a combination. Wilder will get up a couple of times but get KO'd quickly. AJ by KO 2/3 round.
The other outcome is one of those right hands landing early and badly hurting Joshua, then Wilder swarms him and takes him out. The reason I find the other more likely is that AJ has shown he can take top level power and keep going, and that Wilder usually takes time to find the proper opening for his right, time I don't think he has here.