The primary difference between a 7000 SXT and 5600 SXT is size of the internal water passages--the 7000 is a 1.25 in valve internally and the 5600 is 3/4 in internally. The larger passages in the 7000 means it is suitable to higher flow situations and handles larger media tanks as compared to the 5600.
The 5600 is available without "electronics"--and those who are concerned about the durability of electronics find that preferable. All 7000 valves have electronics.
Both are, in my opinion, high quality valves and have a long expected life.
You need to specifiy what you mean by "fairly hard water" and tell us about the size of your supply lines to get a more specific answer whether the 5600 or 7000 is better suited for your situation.