Hello
I'm reading this thread with interest as I own both a side by side and an over and under and whilst I like both, I'd never consider a single trigger on a DG gun so that tends to steer towards (although not exclusively) a side by side.
To hopefully compliment the discussion I've shamelessly copied what follows from a thread on a UK site (Author - HeymSR20). This explains a little of why doubles are the way they are...so to speak
Firstly to principle groups of rifle style and this comes down to very much how they are used.
Firstly British. The British gun trade was primarily focused on shotgun shooting of flushed game, and the British style is very much carrying gun with butt down, muzzle up and tracking the bird with muzzle and firing as butt hit shoulder. When Brits went overseas and were faced with big scary animals with big horns, tusks, sharp toe nails and big teeth the Double Rifle, built in the main by the same makers as the fine shotguns mimicked the handling of a fine British shotgun - fast handling, weight between the hands and balance to enable a snap shot to be taken at a fast incoming or running away big animal - and often in dense cover.
The British Guntrade developed in Birmingham, London and Edinburgh / Glasgow with companies that built everything in house (Purdey, Holland & Holland etc), but with the majority of gunmakers relying on trade suppliers - typically from Birmingham who would supply semi finished component parts that would then be assembled into finished guns. Indeed there were a good number of Birmingham and London makers who would supply nearly finished guns to all the regional gunshops, as well as to overseas Emporiums, where they would be finished to customer spec and labelled accordingly. In the main these were boxlocks shotguns, as well as rifles, but plenty of Sidelocks as well. Generally they were well made and solid, but with different grades of finish. The underlying were pretty much all the same. How time has treated them is the key factor. Not were built in 9.3x74r, but many have been converted to this calibre, and if done well means that they are very shootable today.
Go to the other extreme, the German / Austrian gun trade was much more focused on their domestic market where they tended to hunt big game such as Boar, Red Stag etc as well as birds etc. The style of shooting is very much more deliberate with a deliberate mount and an aimed shot albeit frequently at a moving animal. Shooting tends to be from a stand or high seat, rather than stalking. Hence the germanic style of guns tends to follow more of rifle type balance with the weight well forward to give a deliberate swing. And often there would be multiple different types of game hence the combination guns and drillings where you have multiple barrels often of different calibres and guages. Many - particularly the Drillings and Vierlings (four barrels) make good sense when handled as a rifle, but not as a fast handling shotgun, which seems odd until you realise that things like Black Grouse and Auerhaun would shot sitting, but with the shot barrel.
French and Italian guns tend be somewhere between Germanic and Briitish, with the French much more into wing shooting - indeed the sport of shooting flying game came from the French courts
In Europe there are old centres of Gunmaking - Liege in Belgium, St Etienne in France, Suhl in Germany and Ferlach in Austria all had their own destinctive styles but with lots of different individual gunmakers all producing individual guns. Many have now gone, and many of those left may well dissappear, however some evlovled into major businesses, albeit still pretty bespoke.
Germany - you have Merkel, Blaser and Kreighoff who seem to dominate, but still plenty of smaller makers such as Zimmermann Waffen oHG
Austria - Ferlach - still some utterly beautiful rifles being built by the likes of Home - Johann Fanzoj, Hambrusch Hunting Weapons - Jagdwaffen Ferlach - Used Guns - Barrels - Gebrauchtwaffen, and Peter Hofer Jagdwaffen - Guns - but you will also find many many other Ferlach guns built by makers who are no longer there.
Italy - you of course have Berretta, Rizzini, Fabarm etc who all build good and nice guns. There actions are strong and many do build perfectly good and functional in 9.3x74r. There are though some lesser makers who whilst can build good shotguns seem to struggle with regulating a double.
France - Chapuis seem to be the most active double rifle maker and have looked and handled and they are nice. Dorleac & Dorleac are also nice.
Belgium - Browning are by far the most prominent, they do make good strong double rifles based on the Browning Shotgun action.
Key on any double rifle is Regulation - how the two barrels shoot compared to each other. In traditional doubles the barrels are soldered together and regulation is what it is. ie it was regulated with a certain type of ammo. Change anything - such as mounting a scope or using a different weight of bullet etc wil change the regulation. With most side by side doubles the barrels tend to be regulated so that both barrels will shoot together at say 80 yards. Before that they will start off an inch apart and then converge at 80, and beyond that they will get further apart on a windage basis. With an over and under you don't the windage factor, thye tend to shoot higher as they get down range. Bearing in mind that a double is really a short range weapon - this doesn't matter so much. But for longer ranges a scoped over an under in a mid range calibre - say 7x65R may be a better option. You will have one barrel with shoots well out 250 yards and thats the barrel to use for longer shots. The other is there for when you need two quick shots.
You can change the regulation on an older double rifle, but it's a specialist job involving solder, blow touches and refinishing.
Many of the modern doubles, especially the Merkels and Blasers have a "thermo stable system" where the barrels are not soldered but instead have systems of grub screws where the user can adjust them to regulate to different types of ammo - you are in effect bending the barrels. These are a pain in the arse to set up - 1/4 turn can make a big difference, but once set up its a leave well alone and don't mess with it type system. In my Ferlach 7x65R/ 16bore combination, I have a second liner barrel in 7x65R so I have a double and it uses this same system. As a single shot with the main barrel it works very well and is accurate out 200 yards. Rigged as a double it shoots well enough on a 50 metre running boar target and can both put barrels into the killzone on a pig. But could I confidently take head shots with both barrels on a rabbit at 50 - probably not.
In terms of Value - its all down to perceived value. A quality gun will always sell and hold value. A less well made gun or lessor maker - value will drop through the floor. But they may still work very well. Baikal double rifles are crude - but they work, but you can by them for not a lot f money.
There are plenty of good double 9.3x74r's over and under's by the likes of Merkel, Browning and many of the Ferlach makers in the @1,000 up €3,000 level that will work well. Go to the likes of eGun and you will see plenty. Some of these will be very good, others shot out. What seems to be happening is that the younger generations are having these passed down, but they are following fashions and trading them in for Blasers R8's - pretty much as British shooters are trading in their old fine side-by-sides for Berretta Silver Pigeons.
Hope you found that interesting.
FN
Truly sinister forces at work. 
