Jessie32, since you are considering a timeshare purchase, do learn more at
TUG, as suggested above by Pooh Girl 71. You'll learn invaluable information. The bulletin boards/forums and the "Advice" articles are free. But for someone considering a purchase, you would find the ($15/first year) TUG membership extremely worthwhile. Read all the reviews from owners, renters and exchangers for any resort your are considering purchasing.
Sheraton Vistana Resort is a great place. Since you've already stayed there and you loved it, you know that. As mentioned, TUG has classifieds listings and you may find a good deal there. Remember that some listings are going to be negotiable, both on TUG and with resellers.
Consider whether you want a fixed or floating week or points. Decide this before you search for a good resale. This is an important decision and will affect your longterm satisfaction with your purchase. TUG explains the differences.
If you choose a floating week, make sure it includes all the possible seasons that may interest you in the future. With Vistana, that means you would probably want one that may be scheduled for vacation during Spring Break weeks, all Summer, Thanksgiving and Christmas/New Year's weeks. Some are only good for January, May and Autumn months. While you may not want to revisit during the most crowded time of year, the week will resell more easily later if this is an option. That reduces your risk with your purchase.
Since you like the Falls, that may be a good choice. Did you see the other sections too? The Falls is the only section which has twin beds in the 2nd BR, as opposed to 2 full size beds. If you hope to travel with a large family or 2 couples, you may appreciate the larger beds. Otherwise, go with what you like. It's your money and your vacation. Don't get an every-other-year week. They're harder to resell later. Usually, you can find an annual week for the same price with just a little searching.
When considering costs, include your purchase price (should be under $5,000 including closing costs, maybe less, for a prime week, especially in one of the older sections, like the Falls), annual maintenance fees and annual FL property taxes which are billed separately. Make sure you consider the costs worthwhile.
If you love to return to the same vacation home yearly and you know you want to stay at this resort rather than try different ones each year and especially if you want to travel during popular school breaks, then you will probably love having your own timeshare vacation home in FL. A lot of people will say to buy elsewhere for less money and trade around. (That's what we do.) But it really depends on what you want from your ownership. Read on TUG and decide for yourself.
Here are some reputable timeshare resellers. Remember to offer less and try to negotiate. Look for a great deal and you could save thousands. Make sure the week you are buying is in a section that would please you.
TUG Classifieds by owners
Trish Grossi's Timesharing2000.com
The Timeshare Store
Holiday Group
E. L. Reid
HTH! Enjoy!
Hi. Definately check out TUG. You don't need to buy membership to read the discussions only some parts including official resort reviews are member only. 
