I like stainless steel, although galvanised mild steel will do, I am sure. The usual mesh number is eight and it would not matter a jot whether woven (usual) or welded (unlikely).
Epoxy coated was touted as an expensive, but well recommended screen at one time - it obviously failed when the floor was scorched, by many beekeepers unaware of the limited flame resistance of the material.
Th*rne have supplied the galvanised expanded metal screens for several years and they are good in that the rigidity of the floor construction is enhanced. They were expensive for two reasons. They appeared to have been guillotined from 1.2m wide sheets with the waste of 300mm being fairly considerable (more than used for other purposes by them as they offered 300 x 300 squares for about 50p a throw in the sales) and by virtue of the supplier being one of the 'more expensive' ones around.
Aluminium will be an expensive alternative and some (MB per eg) are now supplying plastic mesh as OMFs with some units. I would not want anything that is flexible, or chewable by rodents, so that was the first thing I changed on the MB poly National floor.
I use the standard woven galv or S/S as supplied by the M*sh Company. Competitive and quick helpful service. Available through epay, if you just want a few. The galv (cheap) is a particularly easy way to make your own travel screens.
RAB