There are so many variables on this. We have stayed at both and really loved both.
Vistana- This really depends on what kind of unit you are assigned to. They have much older less desirable units and newer, beautiful units. We stayed in the third newest (I believe), Fountains. Our unit was very large and the kids bedroom was huge with 2 doubles.We loved the Vistana. We got a unit with the pool literally right outside the door and that is where we spent most of out time. We were in the quieter section of the resort. The Vistana is huge, lots of trees, just very, very, nice. Location is very close to Disney.
Cypress Harbour- Gorgeous resort, the units are all relatively the same in size etc. (I think). Lots of green, walkways, 2 very nice pool areas, plus 1 small. Great clubhouse, lake, bike rentals, paddle boats, etc. Great service. Cypress Harbor is close to International Dr. and Seaworld although there is nothing directly by the resort in the way of restaurants etc. Although, just a short drive away. For us, the one drawback was the fairly small second bedroom. There was a double and a pullout bed, extremely uncomfortable and set at kind of a wierd angle. My DD10 & DN12 at the time, really like to spread out and with all their stuff it was tight. When the pullout bed was out, which was all the time (kids!), it was difficult to move about the room. Would not be a big deal to some, but to us- it mattered. Having said that, I would absolutely stay there again as I did love the resort. Those poor, poor, kids would just have to suffer ha! ha!
Bottom line, depends on what amenities your family looks for.
HTH
Jenn