Its going to be Killer. Killer is a vice captain, Sanji isn't. Killer is a member of the worst generation, Sanji isn't. Killer was on the roof, sanji wasn't.
Sanji is clearly stronger than Killer, but Oda is still going to give Killer preferential treatment if not solely to push the fake rivalry between him and zoro. Look what he did with the captains and how he gave them all equal bounties.
Sanji is a 2nd or 3rd commander, characters like this don't usually get ACoC. Especially when sanji already has another powerful ability, his flames and his genetic powers. But Killer doesn't have flames or an exoskeleton. All he has is his spinning blades, physical strength, and haki, just like zoro.
There's literally no other way to make Killer strong but to give him a haki boost. But Sanji has his flames and genes. Watch him get some sort of equivalent to DF awakening, allowing him to spread his flames to the environment.
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Also, I think the fact that Oda gave Sanji ifrit jambe and his genetic power up in parallel to Zoro getting ACoC is a sign that IJ+exoskeleton is supposed to be Sanji's equivalent to ACoC. Obviously ACoC is stronger, but I don't think Sanji is too far behind.