I've owned both Sinn 556i and Damasko DS30. However, I would say that the build quality of Damasko is better than my Sinn.
I received a not accurately aligned hour and minutes hand Sinn 556i. Sent it back to Europe to claim the warranty and need to bear the shipping cost 50 Euros.
When it came back, the hour and minutes are aligned nicely. I read some reviews online that Sinn have some issues on their quality control.
But still, they made a hell out of a toughest tool watches. Perhaps, it just my luck to receive a lemon one.