Pros and cons of plastic/gravel shed bases vs concrete base
I would definitely recommend gravel for the primary reason that it will drain moisture away from the shed more quickly than concrete. 
As far as the plastic grid+gravel method, I don't really have firsthand experience with that style of foundation; I'm more familiar with installing a treated wood perimeter filled with compacted gravel to place the shed on. There's a pretty good how-to guide at https://www.siteprep.com/how-to-install-a-gravel-shed-foundation/ and they have a video guide embedded as well.