I have a hard time thinking someone can pay over a grand for a 10 animal rack when i could easily build one for under 300, prob closer to 250. its your money, you decide if their good value for you or not.
ultratherms are flexwatt that has a resistor and another layer of plastic.
nothing really special about them if you ask me. if you set each for 91, they will both be 91, no difference. people like to argue "what if the thermostat fails?" not that it hasn't happened, but it takes quite the series of events to get a thermostat to fail, letting current through and not pop a breaker.
I plugged mine in when i first got my rack just to see how hot it got, 147 it topped out at on the hottest spot on the tub.