They are different locations:
SVR: 8800 Vistana Centre Dr, Orlando, FL 32821, USA
SVV: 12401 International Dr, Orlando, FL 32821, USA
I think the biggest difference (other than location) is size - Sheraton Vistana Resort is a fair bit larger than Sheraton Vistana Villages...that means there is more there to do but also means it is more spread out.
The original Sheraton Vistana is a collection of different building styles as some were former apartments. Not all buildings have elevators. The room interiors are nice and consistent and the grounds are lovely and mature. It is a big resort spread out over a large area.
Sheraton Vistana Villages was purpose built as a timeshare and all buildings have elevators. They have a fun pool slide.
I like both resorts. Do a google search and take a look at pictures of both resorts to get a feel of layout and size.
If you book a 1 bedroom at either resort, be aware that they come in 2 sizes - small one bedroom (around 500 square feet) and full one bedrooms (around 900 square feet).