I use a mix of both, with usually more soil, but other people use pure sand, or no sand at all, etc. So you have quite a bit of freedom, as long as the material, when moist, can hold up a tunnel.
I recommend making a few holes at the bottom with maybe some screen hotglued on so that any excess water can drain out if you mist a lot. Otherwise you don't notice but there are 2" of water at the bottom of the bin after several weeks. A few tiny holes drilled into the container solve that problem.