Regarding the 2510SXT, 5600, econominder, 5600SXT, 7000SXT, for flow rate it will have no bearing for your application. The older Kineticos are great valves, but not very high flow rate valves.
5600 econominder, Fleck workhorse. It has been around for over 25 years with little change. The original design was so good that it is still one of my best sellers. I have used it extensively in small commercial applications. Its major flaw is an undersized drive motor, and lack of any electronics for automatic override, diagnostics, adjustable cycles, etc. Very basic valve. This is also a strength, simplicity is not a bad thing.
5600SXT, an updated electronic version of the original. It is a fairly good valve. The electronis are strong, the drive train adequate, it still lacks the higher flows and backwash capabilities of the newer designs.
2510SXT, my second favorite valve. This valve is based on the original Fleck commercial series. It has been updated with a Noryl (plastic) body, and it accomodates the old Fleck bypass. The older brass commercial valves that this valve is based on do not accomodate a standard bypass. Weakness, mechanical microswitches (these last 15-20 years), sandwich style meter ($150) not NEMA rated, other than that, nothing. Strengths, piston drive motor, extra strong piston drive motor should last longer than you or me.
7000SXT, Fleck newest valve, (not including the valves that are being introduced tomorrow during the WQA, I will be in booth 1311 if anybody can make it by), a single valve to handle the smallest house to the largest house. Designed to replace the majority of the Fleck product line. It has massive backwash capabilities, NEMA rated for outdoor use, strong drive motor, no microswitches, ultra high flow bypass, soft water brine fill, multiple plumbing connection, can beprogrammed for brine fill first, very important for people who desire to use potasium Chloride, double backwash, and a 32MM manufold system. The 7000SXT is technically an 1-1/4 valve. Regardless of one persons opinion on this site, the 7000 is not too big for any house. Aesthetically, it is a large valve, so when it is installed on a smaller tank, it looks a little odd. Negatives, cant be used in a parralle tank application using a PTT (this is not an issue for 99.5% of people, but it is an issue Fleck is adressing in the future), Valve is larger than most, making installations a challenge when space is extremely limited (we are talking about installing it in place it should not be installed in anyway, again rarely an issue, but has come up), wont fit in any standard brine cabinet (why anyone uses a brine cabine is beyond me anyway), Attached transformer cord could be longer,
Somebody on this site bought them the first day they came out over a decade ago, had a problem, and forever says it is a bad product, ignore this person. He has also not been directly involved inthe industry for years so his opinion on this valve is meaningless. I do agree that the 2510 was the best valve, prior to the 7000 coming out. There is a new valve being released in a few months, but it is a major redesign that should be given a year to be vetted. I will install a few in my families houses and will give you my opinion when I can give it honestly.
Hope this helps!