Other than one covers the middle of the line while the other covers the side of the line with more offensive linemen or the side with the TE, not a whole lot. Both are called upon to cover non-WR's with the Sam often having the job of shadowing the TE. Both are used as pass rushers. The Mike is usually the defensive play-caller because of being positioned in the center of the field, but not always. There is no reason why the Sam, if always on the field, couldn't handle those duties, especially if being slid over to the Mike in nickel packages. As often as the Saints use the nickel it would make sense for the Sam to be calling the plays if the Mike is the one leaving the field. The Mike is usually the more stereotypical LB, but the Sam also needs some bulk taking on the TE or end eligible which is usually an extra OT.