MDF is easier to work, and a whole lot cheaper. Also with a decent coat of paint/lacquer it will handle moisture well (I left my bench out in the rain for a week while the shed got re-wired and was fine). The downside is that MDF is not as stiff as the plywood of the same thickness, ergo will need more support than the plywood would. Double thickness 18mm MDF will aliviate that issue, I had a sheet of 25mm sitting in the shed for 2 years so I decided to use that!
*yes I work in metric measurements... deal with it, lol