My approach to barrel length ...
Its a combination of weight of the gun and barrel length ...that affect my decision on what to buy. The heavier the gun ..the more it maintains its momentum ( meaning a smooth follow thru ...) because with a shotgun and a flying target...pulling the trigger is the start of the shot, not the end of it...the end of the shot - is the follow-thru. If you pull the trigger and stop the gun ( on flying targets ) it'll mean you're behind everything - in general.
Balance is an issue ...but on most decent O/U's ...whether its a 28", 30" or
32" barrels...on most O/U's the balance will still be between your hands. So it isn't like an O/U with 32" or 30" barrels is nose heavy.
So heavier guns - tend to allow for smoother swings and follow-thru.
Lighter guns - tend to make for "choppier" swings...or what I call "swatting" at a target ...vs a nice smooth swing.
So the lighter the gun ...the longer the barrel I want ...so I don't tend to snap a shot short ...or snap or swat at the bird. The heavier the gun ...the more a shorter barrel will still allow me a smooth follow thru.
28" or 30" are both pretty good optimal barrel lengths on O/U's. If I wanted to error on the shorter side - I'd say go with 28" barrels / but if the 30" barrels still felt "lively" ...then I'd go with 30".
Pheasants are typically an open field, farm area bird...so even longer isn't a big deal ....its not like chasing Grouse thru the trees. But even on real fast - small birds...like Quail / in an O/U, I still like 30" barrels..in a gun that is around 8 lbs. Some guys like to go down to 7 lb guns...because they like guns that swing a little faster.../ to me "faster" means more "swatting" vs taking careful and controlled shots. I'm not saying I don't snap shoot at a really quick Quail flush ...because I do ...but I don't find a Browning Citori O/U like the XS Skeet in 30" barrels to be a clumsy gun ...its a gun I hunt birds with, shoot skeet with and shoot sporting clays with.
hope that helps a little...