20 gauges are usually lighter in weight (mass weight of the gun) and will typically have less felt recoil.

12 gauges are definately superior in the trajectory and horsepower departments. They are generally flatter shooting, and all the while with around a 20-25% heavier projectile.

Personally, I wouldn't really want to be shot with either one, and whatever suits you and whichever gun fits and feels the best to you will be the one that will be best.