It depends on the application. It's hard to beat well placed natural wood in a tank.
However, some of the artificial stuff has really come along. It weighs very little, lasts forever, and can easily be moved around between tanks or taken out without taking weeks to dry and then not sinking again potentially.
Personally, I use both (but not both in the same tank, my experience is stick with one or the other per tank or it will not look right).
This picture is one of my tanks with two artificial pieces at different stages of algae growth.

From here:
http://www.designsbynature.net/products-page/driftwood-categories/tree-stump-003-a/
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