I would not trust any single float switch that I've seen to control an auto top-off, although there are some excellent float switches around these days that I haven't seen. You can use a pair of float switches, one out of the water, so that you have a "fail-safe" when, err, I mean if, the primary float switch sticks. The ones from the guy in Houston look like they're good.

I have an auto top off kit that uses three stainless steel conductors to sense conductivity that I would guess is very reliable. You have to drill three small holes in your sump to install it. The conductivity sensors trigger a solenoid, which turns your R.O. filter on and off. I never ended up using it because I bought the dosing pump for KW, so I'd be happy to sell it for $25.