There are allot of benefiters for both. Wood is lighter and easier and cheaper to install and work with. Steel is almost " Forever". Wither steel is painted or Powder Coated, steel is always stronger. For our rig, the wood flooring was not strong enough to hold all of our equipment. Infact, the average steel deck of 1/8 inch was not strong enough. We had to go with 3/8 diamond plate which is a bit stronger than just plain 1/8 steel.
So for a light weight trailer, to keep cost down, for lightweight items ( 900 lbs or less) that are not necessary to be permanently welded or installed on the trailer, wood would be my choice.
For heavier trailers, 10k plus with equipment permanently installed with stress factors involved, use a cat program.... that will inform you that steel is the best option for those limited situations. These trailers are usually bigger in the weight classification.