Shears are powered metal shear for cutting sheatmetal.
Shears cut sheat metal using a scissor like action. As such they can slightly bend the metal as they cut on the waste side.
Nibbler style shears remove a small amount of material as they cut. A nibbler does not bend the metal as shears do. Nibblers cut slower than shears.
1) Choose a cutting capacity
Shears are rated based on the thickest material they can cut. The industry has standardized to indicate cutting capacity based on the maximum thickness of mild steel the tool can cut.
Choose a model that can cut the material you need it to cut. Keep in mind that stainless steel and other variants are harder than mild steel. Add to its thickness to arrive at the corresponding mild steel number.