A.Theoretically, posts that have been pressure-treated for ground contact can be sunk into the ground. However, it is unlikely that the treatment has penetrated fully through a 6x6. So I would opt for the concrete piers with the posts supported on metal post anchors.

If the posts are sunk directly into the ground, the bottom of the post must not be cut. I have seen a number of posts with the center rotted out because the treatment chemicals did not penetrate completely, and water was wicked up through the untreated center. Post tops should be beveled to shed water, or have a metal post cap added. Any cut ends of pressure-treated material that are left exposed to the weather should be dipped in or painted with a copper napthenate solution.