When I was looking for a trailer to haul two atvs a few months ago, I was looking for one
with the expanded metal floor. I had a hard time finding one big enough for two atv's. When I did find one I was surprised how flexable the whole thing was. You could push
down on the corner and it would flex 5-6 inches. And the guy told me they had problems
with the expanded metal cracking, because of all the flexing. A wood floor adds a lot to
the structural integrity of the trailer. A treated lumber floor will last a long time, I have a trailer that is almost 20 years old, and the floor is still fine.A coat of Thompson sealer every few years will help.
As far as a tandom axle trailer, if all you are hauling is a couple of atvs, I don't think you will need one. If you don't have that much weight on them they can ride rough and bounce a lot, just depends on how stiff the springs or torsion bars are.