I have a 12'x24' building I bought for my brother to use as a shop. It's insulated because it will stay warmer and cooler as the temperature dictates, at a much lower use of watts. The inside temp will also be more stable.
I went with a portable unit instead of a mini split. It is less efficient so in the long run energy cost will be higher. It was much cheaper up front though and very easy to install. I only had to cut one round hole in the wall (maybe 5"?) since I didn't want to use the window.
Given that I didn't plan to be here for more than a few years I didn't think the mini split would pay for itself. In any case it has kept up with heating, AC, and dehumidification needs here in Tallahassee and was very inexpensive to buy and quick/easy to install. If I wind up staying in this house long term it will wind up being more expensive over the long run.