Browning no longer makes the 3.5 Citori. I had one and the stock cracked on it. I later found that this was a common problem with the 3.5 guns. I got rid of it and would not recommend it. Ruger does not produce one though Berretta does. I would stick to a 3" gun in an o/u gun. Just use quality ammo.
A 3" Over and Under will perform as well as a 3 1/2" gun. I personally do not think anything is added with the 1/2". Most 3 1/2" guns are back bored as are 3" guns. There really isn't any difference but the payload a 3 1/2" gun carries verses a 3" gun.