Here are a few things to consider
I own both a 28 and a 20.
20 Ga shells are easier to find and are cheaper.
28 Ga rounds is twice the price of 12 Ga right now - reason I reload for my 28.
Both will shoot basically the same. Just less lead.
Th 28 gun may be lighter in wt
Now I shoot O&U's my Berreta 28 is 4.5 lbs and is oh so nice to carry int he grouse woodcock woods. Far better thatn the 20.
The 28 is a bit light on pheasnats though IMO. So I use the 20 and at times the 12.
But go look at the cost of shot shells and then decide.
JW