I would go back with pressure treated pine or maybe this "rumber" material... It can be installed just like wood...  http://www.rumber.com/trailer_flooring_and_boards/horse_and_livestock/trailer_horse.html

 http://www.redneck-trailer.com/2009/H/H16-H20.pdf

Rumber can be used, but the supports for the rumber are closer than they are for wood flooring.  I looked into getting rumber and decided against it because of the cost of welding in the new supports combined with the extra cost of the rumber.

From the rumber installation guidelines:

Maximum 15" centers.  Recommend 12" centers. 

My trailer has 18" centers, so I would have had to nearly double the number of supports.