I too own a Derby produced by F. Sarriugarte. Although I bought it for quail hunting, I have used it for dove, woodcock, snipe, and duck. It has performed flawlessly and I would recommend them to any upland hunter looking for a quality sidelock ejector at an affordable price. One of the best buys to be found anywhere in my opinion.
Sarriugarte will bring more than the Zabala models as they are higher quality guns. The Zabala models will usually bring somewhere between $600-$1200 (sometimes more). The Sarriugarte models will usually bring between $1000-$1500 (although I have seen them listed for much more). I have also seen 28's and 410's list for well over $2000. I don't know if they sold though.
What is the asking price, if you don't mind me asking? Can you post some pictures?
Here a Zabala Derby...
http://www.collectorsfirearms.com/admin/product_details.php?itemID=10063Compare that to my Sarriugarte Derby...
Adam