We were also thinking about what to choose. In the end, we settled on gravel bed, and did not regret it. The main thing is to properly prepare the site: make drainage, level everything and lay high-quality crushed stone. It is cheaper and faster than concrete, and if something happens, it is easier to redo. We contacted https://www.siteprep.com/ — the guys did everything turnkey: leveled the area, laid gravel, everything was level. Another plus — they offer not only gravel, but also concrete foundations, if you still decide on a more permanent one. They work officially, with a license, and provide a guarantee. They also have services for dismantling old buildings and drainage systems, if suddenly needed. I highly recommend!