I haven't used ultratherm but I imagine it would be more durable as far as moving the tub across it. As far as heat transfer I can't imagine there's much difference in their ability to transfer heat to your tub. Not sure if ultratherm has a more even temperature distribution than heat tape; heat tape can have hot spots. I would agree with you and just do heat tape. Make sure you leave room so you can adhere your thermostat probe to the heat tape. I also would not expect too much of an ambient temperature increase - you may have to find some other way of increasing the ambient temp in your tub.