According to an Orvis book on shotgunning, the biggest effect of barrel length is weight & balance of the gun. I'd concentrate on getting a shotgun that you're accurate with, as per Red Hawk's advice.
This weekend, I tried out a friend's new Citori. Very light and short. An excellent gun for sure, but too light in my hands to keep a good swing on the doves. With my heavier longer (and much less glamorous) pump I fared a somewhat better. The doves can still put "moves on me that Ajax won't take off" but that's a different story.