We are currently on 3 sites, KAS, SDC, and SLS. But from our experience it's mostly based on where you live. I can tell you that here in South Florida SDC is number 1, SLS is second, and KAS is a almost non existent.
For South Florida
1. SDC
More couples, more features, great private, and public events, younger crowd. Can be very X-rated with videos and pix (that can be negative or positive, depending on your view.)
Site can be a little tricky to navigate.
2. SLS
More user friendly. Not as many couples but still strong. Cleaner site as far as what is allowed in the public pictures. Seems stronger up north, we have met a few couples that travel to SFL vie SLS. Negative: site is lacking features, and somewhat old.
3 KAS
Not much experience with it, the small amount of couples we see on that site are also on the other 2. As far as the features it's very similar to SDC.
IMO try them all and see what site has more couples for your area.