The extra space for mowers etc. is a great idea. If I had it to do over, I'd do the size you're planning or something close to it. My
20x20 is wood built on a concrete slab, about 10 yrs ago or so. The concrete was only about $400. I'm going to have to replace some of the siding on the back wall---not now, in about five years or so---because I didn't arrange proper drainage and the bottom of that siding was continuously damp.But if I had to do it over again I'd still use wood, I'd just be sure that the vegetation at the back of the shop was removed periodically to minimize splatter and the constant damp. The mulberry tree I cut down when I built the shop has regrown to its original height...
Depends on how much time and money you have. At this stage of my life (mid-50s) I am discovering that I am less willing to spend my time, more willing to spend money. If I had it to do over I'd probably build it myself again, but not to save money; rather, for the enjoyment of "I did it all myself".
Best -- Terry