Check list on why to use a natural wood such as Redwood over composites:
Composites give off harmful chemicals into the environment–Redwood does not
Composites are unnatural–Redwood is natural
Composites are prone to deterioration–Redwood is not
Composites have a memory and tend to sag–Redwood does not
Composites are relatively new on the scene and not proven–Redwood has existed for thousands of years giving unparalleled service