I'm probably going to regret this.... chiming in on a thread around here is usually a way to get insulted by people who are more noted for their rudeness than their shooting experience/knowledge....But Mr. Stan H. is a very personable and keen shot who has asked a question that I might be able to shed some light on....Some of it ,like others here posted, is personal preference.... but a LOT of it is actual field and competition experience gathered over a period of 53 years, 1967 being my first experience with a Browning O/U.

At the time I got my first one of my own (1968) it was the only high grade O/U READILY available that could stand up to competition level use. When I began registering ATA trap targets in '67 it was the prevailing O/U on most any fields making up 75% of the O/U guns in use.
It was even a higher percentage of dedicated trap double guns.

When I shot my first Columbaire pigeons(a BIG game in Texas in those days)it made up probably 90% of the guns in use.
The leading contenders of the time almost to a man used a Superposed of some type.

When I shot my first Box Birds in the U.S.(1976)they were far and away the most common gun on the grounds of the major shoots of the period. Many being the trap double guns of competitors who shot both games. Back then most every leading Box Bird shooter gravitated to the game from ATA trap. A few exceptions, but Most ATA All Americans of the period crossed over to the ring and performed with similar success.

I shot my 30" Lightning Trap model for my first top score in late 1976 and my first 25 straight 3 months later in February of '77 at the U.S. Flyer Championship, held then at the Dallas Gun Club at its old Royal Ln. location. I used the same gun to win the U.S. Flyer title on those grounds in 1979.

My first trip to Europe to shoot pigeons was to Milan in '79 and the Browning/FN gun in some form made up @25-30% of the guns on the grounds. It was still the most used by Americans, French, and Belgian shooters. Italian guns naturally made up the bulk of the guns used in Italy at the time.

Stan, I don't keep up with the Sporting clay scene , but I do know that British top gun, John Bidwell of "Move, Mount and Shoot" fame won English, World and FITASC titles with an FN manufactured Browning sporting gun. He later took a better paying contract with another company(Blaser maybe).

Although the Perazzi O/Us make up the majority of the guns at American and European pigeon shoots today, the FNs still appear regularly, including the 28" wide rib model used by a noted Portuguese shooter who won 2 World championships and later a Veteran World Championship all with the same gun.

Although I have not competed with one for years now, I still use the guns in various forms almost daily during various bird seasons in Texas. The 20 gauge gun is probably the most dynamic and keenly proportioned of the guns....they just seem to "click".

As to 30" 20 bore FN's, they are custom shop items , and have been for years now. I have a 30" 20 ga. D5G gun. After seeing this thread , I got it out and used it yesterday afternoon. As fine a gun as one could want.... and as you know I have others with more prestigious (costly) names on them.
Just one man's observations.