The same square/box style column we are planning to use is available in cellular pvc (like Azek), or poltruded fiberglass. Final finish will be painted.

The pvc ones have a decent heft when one knocks on them. But I know that pvc also expands & contracts a lot with temperature swings, and don't know how that will affect seams. I've used fiberglass columns in the past and they were strong and well detailed. I don't know about the poltruded ones (where the fiber is pulled through a die), though, and whether they feel solid enough.

Both can be ordered for decorative or load-bearing applications.

Appreciate any feedback--which construction is better?

Thanks, Mark