Wood. Can't shovel mulch and debris off of mesh and some types of debris may fall through the floor. Mesh works fine if you do not do mulching or cleanups, etc. and if it has suffecient support spacing and the mesh is thick enough the sagging will not be a problem. I'd vote wood though. I just rebuilt my trailer and used mesh for the sides (12" tall) and treated lumber for the floor. It's good setup for me.
If you go with wood avoid pine. It will not more than about 4 years due to rot & weathering. Recommend apitong(relative of mahgony) if you can afford it. It a heavy hard wood that has so much resin it is naturally waterproof and termite resistant.
We have trailers we've redecked with apitong and don't show much sign of wear after 8 years non treated. If you use apitong install boards with no gaps; as tight as they'll fit. Within a few months about a 1/4" gap will appear between boards as they dry.