Electric or air shears use two blades and cut the metal like you would with a regular pair of scissors. Tho in short fast strokes. A nibbler uses a punch and die and cuts a zillion little "C" shaped pieces out of the material. This leaves a wider "Kerf" or path thru the material. Nibblers turn sharp corners better than shears. Most are rated for 18-22 Ga. sheet mild steel. I have cut thicker stuff with nibblers.