Smith was a big strong hard punching dude but I could see him being out worked by the smaller Spinks. It happened to Smith multiple times in his career, Hell Marvin Frazier out worked him in Smith's prime. I'm going to take a prime Leon by UD based on his high work rate. Of course Bonecrusher could definitely get him out of there early.
The best post Ali 1 Spinks I ever saw was against Bernardo Mercardo, a big, strong, hard punching guy that Spinks took apart on the Ali Holmes (I think) undercard .. he may decision Smith but somehow I think Spinks gets sloppy and Smith flattens him ..