The system we've seen, that we feel is the best, is a water-powered sump pump. This system is going to cost you a little more to have installed, but if a storm comes and the power goes out, or your primary sump pump fails, this system goes to work.
It works with an electric limit switch and a float and is installed just above your primary pump. If the float rises the pump comes on. If the power goes out, the limit switch releases, allowing water to power the pump and the pump then pumps water out twice as fast as it is coming in.
The short video below explains how the system works: