Definitely plywood. 4 reasons. 1) Behavior when wet (as mentioned) 2) Ever walked on 1/2" plywood? I don't like the feeling of my roof deck being flexible 3) plywood will provide some rigidity to the roof structure to prevent racking in wind. The same way it does for wall sheathing. 4) The greater mass and thickness will absorb more noise from rain provide slightly greater R value.

Although with that last point. IF you really think venting the attic well is critical to shingle life and that shingles are actually cooled from beneath, then you want really, really think plywood. I however believe nearly all cooling comes form the roof surface from radiation and convective currents or wind.