Finally, I'm ready to put in my two cents worth. I really wanted to use these knives in every day situations to give them a fair shake. I hope you all don't mind my abbreviations for this test report.
They are utility knives, in my opinion. As others have said, they are for cutting and scraping rough or nasty stuff, to save your pocket treasures or "jewelry" from serious degrading.
They both cut up cardboard, open taped up packages etc. although the thickness of the Douk's blade slows its progress through cardboard some. The Merc is better at it.
I used the Douk to carefully scrape some rust off an antique H.S.& B. Harness Jack I acquired. Because the Douk's steel is softer, it worked well not scratching the older blade deeply! It sharpens easily, and holds it's edge moderately well, almost as well as the Merc.
The part I don't like much is the ergonomics of either. The Douk is like having a porcupine for a pet! It is uncomfortable to handle! If you grip it hard, the handle cuts into your fingers. It's all barbs; the back corner of the tang sticks up when it's closed and in your pocket, and the back edge of the blade is sharp. Mine, even after some use and oiling is still too tight to open and close smoothly. The Merc is a little better in the hand and pocket, having a more sunk joint, and smoother sheet metal edges at the handles to ease a tight grip. And for tough work, a lockback has its advantages.
Some of my complaints can be fixed or improved somewhat, but as a pocket or EDC knife, there are a lot better choices out there. Rattling around in the trunk or a toolbox, I can see either of these, but give me a nice grip, better steel and blade profile for my pocket!