Depends on the intended use for the gun with 28ga barrels. If you're looking for a 28 as a way to reduce weight when carrying in the field, a 20/28 2 barrel set is almost always the wrong way to go. Most of them are built on 20ga frames, which makes them heavier with the 28ga barrels.
But if you're thinking mainly targets and/or something like doves, where you may be shooting a lot but not carrying through the thick and nasty (like in pursuit of grouse/woodcock or maybe quail), then it may be an OK choice.
