I've stayed at both and like them both.
Differences:
Silver Lake is closer. Only about 5 minutes from Animal Kingdom. The drive is a pleasant, relatively traffic-free trip down Sherbeth Rd. just west of the World. The Fairfield is east on 192, which can be quite congested with traffic. Count on at least a 20-minute drive.
UNITS: The Fairfield feels more like home. It has carpeting (albeit rather stained and dirty) instead of tile and an upholstered sofa (instead of rattan with cushions). Silver Lake is more utilitarian, rather than comfy.
The Fairfield is a stone's throw from a Cracker Barrel. Nice if you like to go out for breakfast. Other nearby restaurants include Macaroni Grill, Chilis & Pizzeria Uno.
A McDonald's is close to Silver Lake.
Silver Lake is undergoing repainting and new construction. Expect unfinished landscaping and torn up areas as well as some construction noise. Call the resort to find out the current construction status. The Fairfield Resort is complete.
Both units were nicely maintained, but the staff entered my unit more at Fairfield. They could get through the deadbolt and enter when I was inside. One time they entered when I was in the tub.
The units in the Fairfield are quieter. The washer/dryer unit abuts an outside wall so you can do laundry at any time without worrying about disturbing the neighbors. The unit is in the kitchen (instead of the master bath), which was more convenient. The Silver Lake units that face the pool are too noisy for me.
I'd stick with a two bedroom only at each resort. Both resorts have large and small one-bedroom units. The smaller one bedroom is a small lock off unit that combines with a larger one-bedroom unit to create a two bedroom. These smaller one bedrooms are too small (especially for three) IMHO.
I liked both resorts. Check out the following web sites for more pictures.
http://www.efairfield.com/inside/resorts/0021.asp
http://www.silverlake.net/
I hope this helps.