AJ by KO is my guess in the later rounds. But Spinks has a good chance at outboxing him and even hurting him along the way.
Even though Spinks is a far better fighter I have doubts about him being able to beat a prime super heavyweight.
AJ by KO. Holmes hurt Spinks here and there. If Holmes hurt him, assuming AJ could catch him equally, a KO is entirely possible.
Joshua by KO Spinks never had the best defense and Joshua hits hard enough to knock him out when he does eventually catch him.