Union Square Greenmarket--lamb
I've tried both, but prefer 3 Corners Field. I've had 3 Corners Field twice (lamb steak once and stew meat the second time), and both times were excellent. The lady at 3 Corners Field is also very helpful in giving cooking advice and providing recipes. I only had Catskill Merino once, and granted it was lamb shoulder, but the meat was extremely fatty and not very flavorful, even after I braised it in a very nice red wine sauce.