Six-inch-deep basins are cheap to make, but they splash and can't hold much. A 9- or 10-inch sink, on the other hand, holds lots—a big plus when countertop space is limited. Keep in mind that an undermount ends up another inch or so lower, which could strain your back—at which point you might consider investing in a basin rack. Shape matters too. You get more volume with square corners, straight sides, and a flat bottom, but soft angles allow for easy cleaning and good drainage.