First off, the originators of the concept called them under/overs, since they intended the lower barrel to be fired first and for the second barrel to be available for a second shot. The theory is that by building the lower barrel directly in line with the mass of the receiver and the support of the stock, there would be reduced muzzle flip ahead of any potential second shot. In my opinion, it works pretty much as intended.
One might point out that a pump or a semi already do a great job of aligning the barrel for reduction of muzzle flip. This is true, of course, but neither of those designs offer more than one choke selection. They both have other characteristics that are a detraction, but I'll avoid triggering those side discussions here. Both when hunting and in competition, two chokes are an advantage.
I confess to an extreme bias in favour of break-action doubles. In the field, I prefer double triggered, straight stocked side-by-sides or U/Os. In doubles competition, the first shot is almost always closer than the second (and when not, you know ahead of time to set the barrel selector to 'top'), so single triggered under/overs are more practical.
This all implies that no single U/O is best for all applications. What had you planned to do with this U/O?