Go with the 28". The longer barrels, 30", are good for trap. The shorter ones, 26", for skeet. The 28" will be a good compromise, and may be the best for clays.
Since the 30" barrel in an OU is the equivilent of a 28" barrel in a repeater, the 30" barrel would be good---the 28", very good. You may, at some point, decide on two sets of barrels, then you'll have everything covered.
Today, many repeaters are 20 or 22" barrels with screw in chokes, and they are quick on rising birds. That' the same overall shotgun length as 22 and 24" barrels in an OU. I prefer 28" on my 20ga Beretta OU (very light at 6.5 lb) for bird hunting. Follow through becomes more of a problem with the 26" barrels, except on skeet, where the swing on fast moving targets takes care of itself. I haven't shot clays (shame on me).
