It really depends on whether the air in the system is a closed loop or not while they are building up the heat in the device. Sand is, at best, going to be about 1/3 as efficient as clay in an open system (hot air goes in, travels through some pipes, then comes back out as waste exhaust). This number is based on experiments with rocket mass heaters I've seen. It could actually be worse- I honestly can't remember if the 1/3 number was for sand, or the large rock (think bowling ball sized) + gravel mix, which was more efficient than sand.
In this scenario, since your input energy is basically free, more storage capacity = more money, and switching to clay could triple your effective capacity.
If, on the other hand, air is blown in a closed loop, you really only risk burning out your fan and resistive heating elements faster (they don't get cooled down by fresh air). In this scenario, 100% of the energy eventually transfers into the sand, so you're really only losing money if you need to spend more on land for setting up additional units.