I'm not sure without looking up but there will be a difference in the spool line capacity. I do know the MM gearing and an extra bearing on the top side of the pinion gear give it a smoother feel when winding.
Worth the extra dollars, not for this guy. The SLX, in all its versions, has been proving to be a strong reel. Many anglers comparing it to the old indestructible greenie. I've been running two basic SLX reels, trying to break them, and they keep on going and cast a country mile.