Like I said, I would start with our articles. What do you want to do with the tank? You'll be looking at two or three small fish without getting into any large equipment. Basic equipment though, a heater, light fixture and some sort of filtration/water pump is really the base line. You'll also need 1+ lbs/g of a live or base rock that you can cycle yourself.
I would go with a shallower tank if you are interested in coral, otherwise the 20 would be better. The biggest struggle of a nano tank, is the smaller water volume, the more your levels will swing with an increased bio-load and environmental factors (heat, evaporation, etc) The extra 5 gallons will make a difference in a FOWLR tank.