With biodegradable spoons and menus made of wildflower seeds, the new Dupont Circle location of salad spot Sweetgreen (1512 Connecticut Ave., NW; 202-387-9338) is undeniably, well, green. But just how sweet is it? Can its Sweetflow frozen yogurt take on nearby competitor Tangysweet? How do Sweetgreen’s salads compare with the ones at Chop’t? We asked a recent lunch crowd to give us the skinny.