20-gauge shotguns are usually lighter and kick less than 12 gauges, which makes them easier to carry and shoot well.
Tungsten Super Shot (TSS) turkey loads and new magnum 20-gauge waterfowl guns have closed much of the gap with the 12 gauge. Be careful never to mix 20-gauge and 12-gauge shells: a 20-gauge shell can slide into a 12-gauge barrel and cause a dangerous obstruction.
Always use only the ammunition marked on your firearm’s barrel or slide, and follow the manufacturer’s guidance.