I sell thousands of 7000SXT's a year, no problems at all. The 5600 is a great valve and has been around since the early 80's, the 7000 is decades newer technology, both are great. The 7000 is a much more modern valve, higher flow rates, and should be a similar price. Either way, you will do just fine. If you have a large house, the 7000 is definetly preferred.
Many online sellers will have one bad experience with a product and they dont want to deal with it again, espeically those dealers who do not actually install the units on a regular basis so they are not aware of how to maintain them properly.