You pick any thermostat out there and there will be people who have experienced failures. Sure, it is easy to claim that one kind of thermostat has more failures then another, but without actual stats to back this up, it is nothing more then someones personal opinion based on nothing.
The reason most people pick the more expensive proportional thermostats is because they simply work better in racks. instead of either being all they way off or all the way on all the time, it actually adjusts the power to maintain a set temperature. This is key to getting proper steady temperatures in a rack.
For your case, with one snake not in a rack, the 500r would be an excellent choice if that is what you can afford.