Well this has really been done to death...
The Spartans will probably be more agile (though maybe not, SM are boss) but the SM will have the strength and toughness advantage. In a prolonged fight both of them will have wounds healed (one by tech and one by biology), but in general it depends on where. In an enclosed space? Cage match style? SM all the way. In a more open area it would be harder to tell.
Squad vs squad though I think SM would win hands down.
And how many Astartes? All of them? Vs. all the Spartans? Yeah, they would annihilate them.
If you go down to the woods today, be wary