Wifi 16GB always seems to get the best ROI for iPhones and iPads. Many buyers don't really care which model they get and price is the driving factor (why they are buying used). The price floor is set by the cheapest models and it doesn't rise very quickly for the more expensive ones on the second-hand market.
People who buy used items are hyper price conscientious therefore they are not going to pay a premium for options that new buyers do. That is why the price gap between used iPads of the various varieties is much slimmer % wise than that of the new devices.
So the 16GB WiFi will return the most $s back for you but never buy something for the next buyer. Buy what you need (within your own budget, obviously) and enjoy.