Recycled Plastic Lumber
There's already been a review of composite decking here, so I won't go into the pros and cons too much. But there is one con that is not mentioned--that is, despite the fact that all materials in composite are recycled, it is made of a combination organic and synthetic materials (wood and plastic). This seems a good idea, but later on, means that the boards will just have to go to waste, as there is no longer any way to recycle the materials to make something new from them when the life-cycle of the deck is over. This means that it only delays the landfill... (Products that fit a cradle-to-cradle design are completely recycle-able--forever, making them the most "sustainable" there is.)
The solution, go for fully-recycled, fully synthetic material! It might feel like you sacrifice strength here, but not with reinforcements of fiberglass, as we see in Bedford Technologies FiberForce Plastic Lumber. The strength is comparable to composite, without the drawbacks (both ecologically and functionally) of wood.
Check it out, it also comes in very wood-like finishes, so you don't compromise looks. It's really worth it--for you, your home and the environment.