Another point, and I'd love to be corrected here, is that with a container of water you're going to get a ton of convection currents leading to a much sharper h

Another point, and I'd love to be corrected here, is that with a container of water you're going to get a ton of convection currents leading to a much sharper heat gradient at the edges, resulting in significant heat losses with the same amount of insulation. At a guess, and I confess I'm not capable of running the numbers, this offsets the much higher temperature delta of sand.