I have stayed at both resorts, and my personal preference would be Vistana. They have lots of pools (various different types, zero entry, waterfall at another etc.). They also have 3 restaurants (as well as pizza hut delivery), which is nice after being out all day - you can go to the restaurant or have it delivered to your room. We like the units as well. I have heard on these boards that the units can vary, but we have stayed there 3 times and have always like our room. No complaints at all. Lots of family activities scheduled every day.
We stayed at Cypress Harbour in October 2003. They had 2 pools, and was never really crowded (could have been the time of year too!). No restaurant, only pizza hut delivery. They did have lots of family activities, none of which we attended because we were too pooped at the end of every day. The unit we were in was in serious need of an update. The sofas were starting to rip, the wallpaper was starting to peel etc. The unit we had also had a huge tree in front of it, and the view was not much to speak of.
My very long winded vote would be for Sheraton Vistana!!