Well, it probably doesn't go 15. Gene and Conn had similarities, but Gene was stronger and better at everything. So Gene outboxes him for 10 rounds and stops him in the later rounds. Or, Conn survives 15 and loses a wide decision.
Conn was a slightly more technically sound boxer, hands up, a little bit quicker etc. Conn by close decision Conn IMO is the greatest pure boxer in light-heavyweight history.