The most popular guns for Trap seem to be actually single barrels as opposed to O/U. Of course there is no rule. I use a Browning BT-99 or BT-100 mostly. Many trap shooters use pumps and auto shotguns. I too use my Browning XT O/U for trap doubles. I have often shot with a Baretta 390 and it smoke them as well as anything. My trouble with a SxS is that there is so much barrel in the air, it is like looking down a sidewalk as opposed to a more standard shotgun rib.
There are a lot of Perrazi and Kreigoff shotguns being used, I love them, shoot them well, but I just have a hard time wanting to part with that much money. I am looking at a Rizzini right now, a buddy has one and it is really impressive for the price., I will wait a bit and see if he has any breakdowns before I buy one. One thing about Brownings, I haven't seen anybody get off the line because of a malfunction, but I have with some of the VERY pricey guns though.
The bottom line is, you can get any gun mentioned in these posts and be assured you have a good gun, some just more costly than others.