The 28 gauge fans, of which I am one, have an oft quoted mantra which is just the reverse of L.Brown's. Anything a 20 will do, so will a 28. And I have to disagree with L.Brown in that a 28 is not necessarily a short range target gun or for small, relatively fragile birds. With the proper choke, shot size and appropriate load (and a competent shooter) a 28 will do anything a 20 will do.
With the expense of 28 ga. ammo and the possibility of a slightly heavier gun, the only advantage I can think of, considering the scarcity of 28 ga. ammo, is the reduced recoil.

I love the 28 but if I were in your situation I might stick with the 20.