The 28 is a pleasant novelty searching for a niche, while the 20 cuts a wide swath with the best of the attributes of the 28 with a much more versatile capability.
I don't find a gun less than six pounds shootable; my shooting falls off significantly with very light guns. This is the achilles heel of the 28. When made light as is appropriate to its size, the guns are voluptuous but too light, IMO. Heavier, one might as well have a 20 (and as others here point out, the 20 loaded down is more efficient than the best 28 guage load, and has a top end the 28 can't touch).
Many 28s have come and gone with me, and I liked them, but logic ultimately kicked in and my six pound 20 won the day, and still does.
But I must admit to weak moments. This one could seduce me (29" Piotti BSEE, 5#13 oz.) pretty easy. This isn't all about logic, is it?
http://www.williamlarkinmoore.com/images/_MG_2103.JPG
