This isn't about who's greater p4p between Charles and Spinks, it's about who'd win at cruiserweight.
Spinks would win. Better jab, longer arms, taller, faster hands and at least an equal infighter. His peekaboo gives him a defensive advantage too. When Charles gets inside he'd be punching gloves and elbows.
I got Charles landing some very vicious counter-uppercuts on the Spinks right in the mid-late part of the fight. Very hard to say without more information - over twelve, corner is in the air, over fifteen, Spinks has been boxed in a bit by Ezzard's firepower and is consistently winning exchanges. If I were laying money it would be on Charles over any distance above ten rounds.