It would be an awesome fight. Both the Commandos and the Spartan would have to pull out all the stops in order to win. Commandos would probably work together to flank the Spartan, corner him, limit his movement and then go for a kill. While the Spartan would try and stick to shadows, working to terminate the Commandos one by one.
I always though Spartans, if anything, compared to ARC Troopers more than they compared to Republic Commandos. Arc Troopers and Spartans perform alot of the same type of missions.