This is a tough one...one gun being more of a sporting gun and another being more for hunting. What's even tougher is you mention a "birding" gun as opposed to specifying what kind of birds.
Generally speaking, sporting clay guns are a bit heavier. A heavier gun will help you to swing and follow through a bit more - that's what a gun designed for shooting sporting clays is about.
On the other hand, a "birding" gun should be a little more specific to the type of hunting you want to do. For upland game, a lighter gun is more desirable because you're going to be walking quite a bit with a gun in your arms. For duck hunting, a heavier gun is more desirable because you're probably going to be making longer shots that require more velocity (longer barrel), will help you with your follow through, and that will dampen some of the heavier recoil loads.
It depends a lot on what you want to do with it. If you're primarily a hunter, then shooting clays with a hunting gun will only benefit you in the field. If you're primarily shooting clays, then get a target gun.
It all depends on what you want to do.