I like the Visi Therm Stealth heaters... have been using them for 5 years or so...
I aim for 2~3 W per gallon... more than that simply adds risk to your fish in the event of a heater getting stuck on...
There has been an ongoing debate for many years whether one or two heaters is a better idea. To my understanding... if using a single heater controller, two is the best way to go... if relying on the dial on the heater itself, one heater is a better idea (as it's extremely unlikely they will ever be set at the exact same temp).