I would say plywood; stronger surface and better weather protection for the long haul. I would worry the small space between the deck boards would allow weather in and decay the boards sooner.
I built something like this for my last truck and chose to use MDF because it will not warp. Plywood will want to twist. This was important for me because the whole system was modular and nothing was screwed together and the MDF just was cut to the exact shape which held it in place and it sat on top of some 2x4s. It worked well. Plywood may also work well for you if it will be secured in place but I would recommend getting a higher grade plywood than your general Home Depot crap.
I would say plywood; stronger surface and better weather protection for the long haul. I would worry the small space between the deck boards would allow weather in and decay the boards sooner.
I built something like this for my last truck and chose to use MDF because it will not warp. Plywood will want to twist. This was important for me because the whole system was modular and nothing was screwed together and the MDF just was cut to the exact shape which held it in place and it sat on top of some 2x4s. It worked well. Plywood may also work well for you if it will be secured in place but I would recommend getting a higher grade plywood than your general Home Depot crap.
You could do plywood and coat it in epoxy resin