I'm in the multiple, smaller heater camp. Two 100 watt stealth heaters should do the trick for you. If one fails in the "ON" position it shouldn't fry your tank by the time you realize there's a problem. The stealth heaters have a plastic outer casing vice the conventional glass heaters so there's less chance of accidental breakage that way.
Even a Ranco controller can fail and no matter how robust, it's a single point of failure. With two smaller heaters you have some redundancy.
To be honest I have no personal experience with Ranco controllers but most newer aquarium heaters have electronic switching vice the older bimetallic switches to turn them on and off. I'm sure a Ranco controller uses a relay of some sort as well, whether electronic or mechanical and none are completely failure proof.