I’m a fan of using the strips, both because it’s flush to the wall (unscrew other hardware if needed) and because if it’s not perfectly level you can pull it off and reattach to the backing strip. Works best if you apply to the frame with an inch or two of space from the edges so they’re not visible after adjustments.

If it’s a heavy frame you’ll have to resort to finding a stud to drive a nail in, so pay attention to the weight limits on adhesive strips.