I have an 870 as well and I also do a lot of bird hunting. I'm telling ya you should get the 28 inch. Theres nothin better, I think it really helps with my swing through. Never had a 26 but I don't think I'd change over at all.
the ease and speed of your swing will change with the different barrel length. depending on your shooting style, one of the barrels would be better for you than the other. i've got a 26" on my semi. had to borrow a 28" barrel for a month and had a hard time hitting anything with it. i was shooting behind the birds.