4seventy:
Tradition would suggest that SxS shotguns are more suitable for game-shooting than U/O guns, and those of us who like our rifles to match our guns might therefore prefer SxS double rifles for the hunting field. These days, many gunners are starting life on clays, and taking their preference for U/O guns into the field. This is gradually changing the supremacy of SxS guns in the field, and may be expected to permeate double rifles in the future.

For me personally, I shoot with both eyes open on both shotguns and express-sighted doubles, and can't stand the left-eye peripheral view of the lower barrel on an U/O gun or rifle: it really distracts me.

Technically, I believe that U/O rifles have the dis-advantage of larger opening angle, which is NOT trivial for dangerous game. The large-head, rimmed cases needed for DG in an U/O rifle dictate opening angles approaching 70 degrees, which is much slower and gives more opportunity for things to go wrong, or the shooter to fumble under pressure.

The U/O design has one important advantage for DG though, in that the lower barrel produces significantly less barrel-rise from recoil, allowing a faster second shot. I don't think total weight or strength (in either design or materials) is an issue these days, though it may have been a significant disadvantage of U/O big-bores in the past.

...and since this is just my opinion, SxSs rule!