The hand built O/U was an expensive proposition and still is. It was generally built in high original quality grades. There was nothing approaching the volume, and specialized industry to support it, of the SxS in Britain. The Browning Superposed was something of an interim step in combining a mostly machine made gun with enough hand work to make it widely acceptable for original quality and price.
Looks to me like the SXS did not get a manufacturing technology update because it was an existing product with dedicated factories in existance. The O/U appeared at a time wherein it got the advantages of big leaps in metal working technology. Cheap autos and pumps were the bain of the SXS, but they left open a market for a "better" gun and the O/U could fill the void at an acceptable price. Fortunately for the O/U, heavier guns make generally better target guns; clay target shooting didn't really hit its stride until post-WWII. The last 50 years has seen the same kind of creativity lavished on the O/U as was the case for the SXS in the last half of the 19th century.
I find no significant difference in the artistry of fine SXSs and O/Us. I find no significant difference in the shootability of either pattern of gun when in equal purpose built configuration. The current handicap for the SXS is playing catch-up in manufacturing technology - we will know if this is possible based on Tony's bold RBL experiment and Ruger's Gold Label.
In the mean time, I plan to shot game guns when I'm out for fun and target guns should I get serious. BTW, my Ithaca NID 4E (trap configuration) is fully competitive with anything that shows up at the club where I shoot. Hal Hare pretty well settled the issue for skeet with his M-21. IMO, shootability is more about first learnings and fashion that real advantages.
