I voted for 28 gauge as it'll likely be my next shotgun purchase.
3/4 or 1/2 oz of shot will save $$$ on reloading components
I load 12ga down to 1/2oz. Of course such light loads might not cycle a 12ga semi-auto, but in a pump they are fine. It does save money, I only need 12ga hulls, and have a lot of flexibility with four 12ga 870s.