No pros and cons, really: more of a matching the way the gun handles to the way you want it to handle.
A shotgun for wing and clay shooting is like a golf club. You want its natural swing speed and characteristics to match the way the target flies.
The differences are more obvious with upland birds.
My favorite gun for California Quail (prolific in this part of Idaho) is a 26" 20 Gauge O/U that weighs under 6.5 lbs. The birds are FAST, they fly low and drop, and even when you're hunting with a dog, desert quail tend to flush out with no warning. The gun shoulders FAST, the safety comes off unconsciously, and the barrels swing quickly to match the birds. Carry a heavier, longer 12 Gauge ahd you can kiss off bagging many, or, often, any California Quail. Chukar and Huns give a little more of a window for the shot, and tend to be a little farther away. A slightly slower, longer gun like a 26" 20 Gauge semiauto seems to work well. For pheasants, it depends on the way they're flushing. For doves, the ideal gun is probably light but long, like the Benelli Cordoba 20 Gauge, which is made specifically for dove shooting.
For incoming ducks over decoys, a shorter 12 Gauge can be the ticket: point and shoot. For passing flocks of waterfowl, a longer 12 Gauge or even 10 Gauge can swing slow and smooth, to help make those kinds of shots. Even before steel shot, "goose guns" tended to have 30" barrels.
An 18.5" HD gun is made for quick pointing, but not smooth swinging. On passing targets moving fast, it is not a great choice -- but with effort and practice, you can hit a passing target. After doing it for a while, you'll get a longer barrel, or a different gun for hunting. Note that good shotgunning on moving targets involves moving your entire upper body with the gun. A longer gun makes that much more natural.