I've had both. I had a Yardmaster (self build) steel shed in my last house. Fine for storing stuff like bikes, lawnmower etc, but that's about it. The concrete one I have at the moment is great as a workshop, is much easier to heat when necessary, and if I wanted to use it for something like a home gym, then it would be streets ahead of a steel one, with a little more work in drylining etc. It is a lot easier imho, to customise afterwards, e.g. putting in dividers, adding a door etc.