I had a couple of rotten sheds that I replaced with a much bigger
summer house last year (work still in progress as you can see from
some of my recent posts).

I specifically addressed the issue of the summer house base (let's
call it big shed as the structure is the same), both looking at what
happened to my rotten sheds , to neighbours' sheds and to the
extensive and disparate comments/literature you find online.

Metal: depending where you live, unless it's properly insulated you
can have problems of internal water vapour condensation in the cold
season, as metal ends up being colder than the surrounding
environment. You could end up having an oven in the warm season (even
if it doesn't look very warm outside) because of solar radiation being
nicely absorbed by the metal.

Plastic: UV tends to degrade many plastic materials with time so I
wouldn't put a bet on it lasting 20 years... unless it's explicitly
certified to be UV-resistant.

Wood seems to be the longest latsting option unless water gets its
way over, under, around... So that's the material I chose in the end
(also because it's the one you can more easily modify).

The outcome of all this ruminating on personal and other people's
observations is that putting the joists (even if pressure treated)
directly on the soil ( levelled, graveleld, etc it's still soil) is
pretty bad. What is always recommended is something along

- level the ground
-put damp proof barrier (thick black polyethylene sheed) on ground (or
below if you lay cement)
-put slabs on barrier
-now you can put joists on, although I would further recommend you lay
joists on bricks to make sure joists are not directly touching the
slabs and getting any stagnating water for long time

Now your shed floor would be seriously safe...

Good luck