52621 I don’t have it hard wired yet so until that’s done I can’t tell you much. It arrived in good visible shape about 2 weeks sooner than originally indicated.
53021-hard wired in. Heated up from 68 (when filled with garden hose) to 100 degrees in about 5 hours. Exactly what we were looking for.  Easy to operate.
My only complaint is the access door needs a better design than the 6 screws that hold it-maybe put it on hinges.