I have put all of mine on pallets so far. I thought that would help reduce the likelihood of them being invaded by fire ants, but that didn't work. The ants still moved in to two of them. I may omit the pallet on the next few and see if there's any noticeable difference.

Without the pallet constraining the size, you could increase the diameter a bit and get a couple more air vent tubes in there, to offset the lack of airflow from underneath.

I suspect you're right about soil contact helping to increase the diversity of the life in the compost. It will be an interesting side-by-side experiment.