Bought a new rifle

[JFellows]

I'm not an expert, but I think Side by Side guns generally use two triggers...O/U (at least in this case) uses a single trigger to fire the shells in both barrels. The gun is cocked when you fold the front end back in to close the back of the chambers (like with a Side by Side...I think), but then you can pull the [only] trigger twice (to fire twice) before having to reload.

[Rmac58]

Not sure on the number of triggers on a O/U.
I'll agree with Eville, personal preference. You aim the SxS in the middle of the two barrels, so I would go with an O/U. My current shotgun is a pump.
They do make semi auto shotguns, but they are very pricey, the newest from Baretta, a break open semi runs $3200.
Some companies make a three barreled rifle, with three different calibers, none are shot shells tho.
I don't hunt, but folks I know that do, use primarily a shotgun of their own want.
Handguns are the biggest crime problem, I'm getting another one, as it's become a hobby.

[Kenyth]

Shotguns were the first powder based hunting weapons IIRC (musket). Because the ammo is a bunch of lead pellets, the barrel tolerances aren't exacting. A big plus for those lacking a machinist shop. Not only that, but before the days of automatic action, multiple barrels was the only way to get more than one shot out of your shotgun.

The original intent/advantage of the more recent double barrel shotguns was hunting birds. The first shot is fired from a barrel with a modified choke. This gives a wider shot pattern at closer range. The second shot is fired from a full choke barrel, which gives a tighter pattern which spreads out at longer ranges. The logic being that, after firing your first shot, the missed first target or new secondary target will be on the run and thus at a longer range.

Clay pigeons are a way to simulate and practice hunting birds. Skeet is a sport based on clay pigeons that the wealthy like to play. Expensive and ornamental over/under shotguns are typically marketed to this group.