The adjustable circular hole cutter is the easiest to use. It requires a cordless or electric drill (not included), and will drill its own 1/8 inch center hole. The sliding/locking center pivot pin has a built in scale to cut precise holes from 2 to 12 inches in diameter.
The Drill-powered shear will cut straight or slight curves, as well as making 90 degree corners. It requires a half inch pilot hole to get started and is easily attached to your cordless or electric drill (not included).