The key here is to think about the materials that are in that "Composite". Usually, this is a combination of plastic, often much of it recycled, but not all, and sawdust or wood fibre. Now, the unfortunate thing is that by combining the two you have something that can be neither recycled nor composted when you are done with it. So, while it has very good durability and ease of maintenance in its favour, the end-of-life question remains problematic. So, that plastic can go nowhere else in terms of recycling, and the organic matter of the sawdust is now bound up with it.
So, while it may look like a sustainable product I am not a fan as it has zero chance of going anywhere other than a landfill when you are done with it. In addition, the way it is constructed and held together impacts its durability and lifespan, and those components may deteriorate and fall apart more quickly than the decking itself, shortening the life of the deck significantly.
What about the shiny PVC materials?
Well, PVC is plastic, and among the plastics, it is one that contains, and releases phthalates, which are endocrine disruptors. These are the nasty compounds that interact with our hormone system by either mimicking hormones like estrogen or testosterone, blocking them, or altering their function. This is harmful to humans, especially during fetal development, and to other organisms. So, while we are not likely to have much exposure to this type of use of PVC, I think it's best to reduce the use and production of this harmful material.
So, in the end, my top choice is naturally weather-resistant wood like Cedar, Larch and Hemlock if it is FSC certified, that is, sustainably harvested. But, if natural wood is not suited to the project or your budget, any pressure-treated wood on the market is safe when appropriate precautions are taken. Don't inhale the sawdust and don't burn the sawdust or end cuts, and certainly don't allow the wood to come into contact with food. The safest type of pressure-treated wood is called MCA (Micronized copper azole) and is dark brown. You can find the type of treatment on the tag at the end of the boards at the retailer, and check the resource guide for all the details.