IIRC, the Model 7 is built on the old Model 600 action, while the 700SA is the short version of the 700 action, made for short cartridges. Remington has gotten a lot of mileage out of both actions. The 600 action began as the XP-100 pistol in 1963, then the action was used on the 600, 660 and Mohawk rifles, then on the Model 7. It is a shorter and lighter action than that of the 700 and was made only as a short action. The 700 action was made in two lengths, but the short 700 action has more in common with the long 700 than it does with the Model 7 action.
Both are fine actions, but I really like the Model 7 in 7mm-08, which I think would be an excellent choice for medium game. Someday, if I save my pennies, I might get one. (I would love a full length schnabel stock in solid walnut, but they don't make it that way.)
Jim