I would say this about the 28 ga: It's a wonderful little gauge that can do pretty much whatever the 20 can do, within a range of about 35-40 yards, excluding waterfowl, and possibly wild pheasants. Not to say you can't take waterfowl or wild phez with it (I have taken pheasant, and know a few who regularly take both ducks and geese with it), but that the 20 has a decided advantage, IMHO. I personally have no interest in shooting waterfowl, and have decided to use "more gun" on wild phez. It works wonders on preserve phez & chukar, as well as dove & quail.
However, there is one thing the 28 can do (not all of them do!) that a 20 can't, and that is carry lighter than a 20 ga. To the gent who can't shoot an under-6 lb. gun well, the 20 is indeed a wise choice. I, on the other hand, shoot my 5.4 lb 20 ga very well, and would love a 5 lb. or so 28 ga. I know they are out there, I just haven't been able to drop the $3500 it would take to get one.
I agree with the philosophy that if you're going to carry a 20 ga sized gun, you might as well take the advantages of the 20 ga round, too.
Having said all of this, if I had to choose one and forego the other, I would choose the 20 ga, simply because of ammo availability and price (I don't reload). Fortunately, I don't have to choose and can shoot whichever has my fancy at the moment!