I've hunted pheasants on preserves and in the wild with o/u and sxs in 12, 20 and also used 28 gauge o/u on preserves.
Here's the thing about pheasants. They are dumb. They don't know whether you are using an o/u, a sxs, what the gauge is, how much your gun cost or whether it has fancy engraving. Pheasants just know if the pellets hit the mark. Nothing else matters.
The gun to use for pheasant hunting is the one you shoot best. Someone who shoots clay targets all year with an 8lb. o/u and then chases pheasants with a 6-1/2lb sxs won't do as well as if he'd taken his usual gun or one closer to it. If what you shoot most often is a sxs then that is the gun to use and it is nonsense to suggest that one configuration is better than the other.
Secondly, clay targets are not live birds. Shooting clays is good practice and lots of fun but clays follow a predictable path, live birds don't. Also, unless you walk around with the gun mounted on your shoulder or in the ready position then much of it doesn't apply.
After trying other guns I settled on a 12 gauge sxs with a straight grip, double triggers and an auto safety. I practice with it leading up to the season but in situations that more closely resemble hunting conditions. On a skeet field or sporting clays house hold the gun the way you'd carry it in the field, safety on and have the puller throw the target without calling pull or you knowing where it is coming from. This prepares you for the "surprise" of the flush.
The more you shoot the better you will be. The more your clay practice resembles hunting the better you will be. The more familiar and practiced you are with the gun you use the better you will be. Shoot the gun you shoot best. Don't worry about whether the barrels are up and down or beside each other.