Personal preference would be:
Concrete floor with polythene membrane below 30mm screed. Finished floor above surrounding land by , say 75mm. Brick course(s)to add headroom and discourage rodent entry. Build in rag-bolts or coach bolts to bolt down shed for security and hurricane resistance! Depending on how you wish to use the space, maybe racking or a bench within, to try to keep the floor clear.