The Boaphile thermostat IS a Ranco. Rancos, Johnsons, and Helix were all designed for industrial use, so they're much more reliable than unregulated pet brand models.
Ranco is just the basic thermostat. You're able to specify a differential. So I could have the temp set at 90 with a differential of 5 degrees. That means if the temp ever goes down to 85, the thermostat will automatically kick on the heat until it hits 90. Then the heat turns off until the temp goes down to 85. But again, I can change the differential to 1 or 2 degrees. Boaphile has a great description of it on his page.
Johnson costs more than a Ranco, but people like the smaller size. It also offers more features, which I think includes day-night timers, but I could be wrong. There are more details on dsreptiles.com.
I know nothing about Helix, but it's the other high-end thermostat people prefer. You can check it out on beanfarm.com.
The thermostats only come with one probe. I just keep the probe on the top cage since that's the one that would get slightly warmer than the rest. For the most part though, there isn't too much variation. I still use a temp gun to check on the cage temps, which I suggest you do no matter what type of thermostat you use.