In an effort to reduce costs, some composite deck lumber is made hollow. Other types are solid and of similar thickness and width to milled wood lumber. Which one is best? That depends. Hollow versions cost less, but they also require end caps and other details that make the result look quite a bit more unlike wood than solid composites. If you have a client who appreciates routed edge details and a look that’s as close as possible to wood, then solids are the way to go. If keeping the budget in check is top priority, then go with a hollow product.
One important thing to realize is that composites have not yet evolved to the point where they can take over from wood in structural applications. For load-bearing posts, joints and beams, solid wood is still king. Why? Nothing yet compares with the amazing ability of wood to resist bending under consistent pressure over time.