Plywood gives you the best bang for your buck but there are many quality levels. Probably the best for your application is microlam, many thin layers of wood bonded with a waterproof glue. If you want to really reduce weight, then go with one of the engineered solutions like Baltek
http://www.baltek.com used when you need really good strength to weight ratios, usually less than half the weight of plywood while maintaining the strength. The first product, Baltek, was pretty simple, just a couple doorskins, thin plywood, bonded to a balsa wood core. The newer products that they have are entirely man made so they will be more durable, and probably more suitable. They would be similar to the foam core floor of your rooftent. The idea is that the core doesn't give you much strength, so make it light out of foam. The stressed skin is where the real strength is, same concept as an I beam.