With a plastic tub, a simple on/off thermostat will work fine enough, so long as the temperature of the room where the tub will be located is stable. An on/off stat will have trouble keeping up if the room is drafty cold, very hot, or varying constantly anywhere in between.

You could go the extra mile and get a proportional thermostat such as a helix or herpstat. I didn't really "get it" until I switched over to a proportional. They rock.