I'm not too sure how one can technically tell a better quality cast iron from another....But, from personal experience, I can say I have 5 different machines with cast iron tops/ surfaces ( all different manufacturers).
The worst is my large foor standing Axminster bandsaw, by far.
Followed by my Charnwood Router table top.
Then my smaller Record BS250 bandsaw table.
Then my Hammer A3 31 planer thicknesser tables/ bed.
And finally, the best of the lot, is my Startrite Pillar drill base and table.
All of them will rust if not protected in some way, but as far as I can tell, the Hammer and Startrite are superior to the others and tend to need far less surface maintenance.
Edit.
I would never cover any of my machines in a plastic sheet, as all you're going to do is trap any moisture and prevent it from escaping.....It's going to exacerbate the problem, not solve it!