Me, glasses mounted for the reasons already mentioned: Wide field of view, less/no vibration, can scan behind by turning the head a bit. Current preference is the Take-a-look Compact mirror.

BTW, the focus in a mirror is basically the same as looking without the mirror. You focus on the scene, not the mirror. Bend the mirror mount or otherwise arrange it so that the mirror is up and to the left, out of your direct view.