I have had good luck with the solid state style switches. I have a bilge sump that is too small for a submersible
pump and a conventional switch, so solid state or the microprocessor controlled
pumps are the only options.
The only drawback I have found is that these tend to stick, metaphorically speaking, when something in the bilge causes bubbles or foaming, or if I slack on maintenance and bilge scum builds up on the outside of the sensor. Basically, it is possible to cause these switches to activate with enough moisture near their surface. Cycling the
pump on and off manually usually fixed this, followed by a cleaning. With any bilge
pump, make sure it can run dry without damage. I think most of the submersible
pumps can.