28ga.
i hunt a great deal with a 28 ga. my go to goose gun is a beretta o/u and i shoot pretty fast steel through it. love it. my bird gun is a sxs with screw in chokes and for pheasant i shoot lite mod. and imp. mod. i hand load for it and shoot a 15/16 oz load at just over 1500 fps and use copper 7.5's. at that speed they have about the same energy as #5's in a regular load. really, really deadly and it only weights 5 #'s. i also have 5 20 ga. but rarely use them. as far as cost of shells go, worry about the price of gas and your beer. forget the shells
cheers