Single. Easier to move around off the vehicle and since the vast majority of your trips are 20 miles you don't need a tandem.
I've had two flats in 30 plus years. I don't have a problem with single axles. I also carry a floor jack in my tool box but I assist others 20 times for every time I need it.
It sounds like you'll need to replace the tires from age before they wear out. A single will easily go 15 thousand miles on a good set of tires. A tandem almost twice that.