I don't believe the check in days are completely cast in stone. What I mean is, DH, DS, DD and I are arriving on Sunday 7/11/04 and we're reserved in Cascades this year. It doesn't hurt to ask about specific sections regardless of checkin date and if you have a number of villas reserved for a family reunion or something, you should mention that as well. I have found the reservation clerks at Vistana to be very helpful and if there is a way to put you together (or at least close) they will.
In terms of units, Cascades is the newest, tend to be the smallest (but not too small, just small in comparison to other phases) and are centrally located. Also, they are taller and have elevators. Lakes is second newest, and one of the larger size units, very lovely, but not as conveniently located. When the children were smaller we stayed in Lakes. We loved the unit, but had to take the car to the pools we preferred, which was a drag with small kids. Fountains II is the 3rd newest. Basic floor plan, but even less optimally lcated than Lakes. Then is Fountains I. Well located, still newish. Then Spas (if I'm not mistaken).
We stayed in Spas in July of 02 and were very happy with it. Like all of Vistana Resort it's decor is "Floribbean" so a little cheesy, but what you would expect to find in a Florida vacation resort (I think). We were right across the "street" from the general store on the second floor. We were completely satisfied. I know that there are some posters that insist you can only have a good time if you are lucky enough to get Cascades phase. But, I disagree. We've been in Lakes, Fountains I, Fountains II, Cascades and Spas. BIL and Sister have stayed in Lakes, Fountains II, Springs, and Falls. We all have had excellent vacations at Vistana year after year. (I know I'm gushing, but no, I don't work for Vistana!)

We love us some Vistana!