We use Rancos and Johnsons and both seem to regulate temperature equally well. The Johnson has temperature offset (night drop) capability and to my knowledge the Ranco does not. The Johnson also has a sensor failure operation, where you can elect to turn the power off if a sensor failure is detected. The temperature differential feature is one area where the Ranco seems to have an advantage. When the Johnson is set to a 1 degree differential, the temp must drop 2 degrees before the power is turned on. With the Ranco, the power is turned on after the temp drops 1 degree.

If you're definitely going with an on/off thermstat, I recommend the Johnson over the Ranco. You will probably pay a few bucks more for it, but in my opinion it's worth it.