Anything Herpstat.
Herpstats are the best in my opinion. They have safety functions that turn the unit off when it malfunctions instead of leaving it in the 'on' position to make the heating element go to max temp and burn your animal. The higher level Herpstats also have alarms that sound if your temp drops too low or too high.
And because they are a proportional thermostat, they keep the temps steady by regulating power to the heating element. On/Off style thermostats(Hydrofarm or Lower level Vivarium Electronics) turn the heating element on when it's too cool. Once it hits a set temp, it turns off. This causes a temp swing of +/- 3 to 5 degrees.
When you use flex watt on a rack do you have to have a probe from a herpstat on each tub, or just on some part of the strip of heat tape?