Well actually as far as what they do and how they do it, they are exactly the same. I actually prefer the Johnsons as they are smaller and have a green LED on the front to let you know that the power is on or off. They also have a metal probe as opessed to the plastic probe ont he Rancos. The metal probe makes them respond faster to a temp flux.. I have one Ranco in use right now and it is controlling the room heat the rest are either JC's or herpstats... Shoot me an email if ya like if you need any futher information.. Hope this helps,
-Matt