Spinks UD on superior power, speed, consistency and overall athleticism against Sharkey's superior ring generalship
Spinks before his knees started falling apart was a lot more consistent so I’d have to with Spinks as the favorite, but if Sharkey shows up focused it could go either way.
Technically speaking, Sharkey would be a cruiser by modern standards. So if you think that Spinks would have done well or perhaps even been an ATG at cruiser, then you have to give him a good chance vs cruiser sized heavies.