Lifetime bronze benefactor
Barrel length is very subjective. I won't own anything under 28". My two favorite quail guns are 30"s (it's the choke you know). My favorite waterfowl gun is a 32. Anything less than 28 feels to me like trying to shoot a flyrod.For hunting upland birds, I prefer 26" barrels, but 28" would be OK if the balance and weight felt right. Shotguns have so much to do with feel, weight and how well they fit. As your experience and skill level rise, these things become more and more obvious, or at least they have to me.
I have a friend with a Browning Citori White Lightning, English stye stock 12 bore with 24" barrels. It's not very often a bird escapes his aim. I'd say shoot what feels best for you.
My current favorite new production upland guns are both 20's. My Rizini Br 550 is a SxS and the CSMC Inverness is an OU. Both have 30" barrels. Unless I simply want to use one of my classics - which I do a lot - these two handle any upland duty.