The Fleck 5600 has been available for many years as it is a good, reliable and well made valve. The SXT is the digital control for that valve whereas that same valve is also available with an older mechanical metered control but, it is close to the same price as the SXT so you may as well buy the SXT.
Fleck now offer a newer digital valve, the 7000 SXT, which is suitable for a broader range of softeners as it provides less flow restriction than the 5600. Either valve is appropriate for your situation.