The most expensive brush is sable, made from mink. They are very expensive and lovely to work with. They will last a very long time if cared for. These brushes out surpass any other brush for watercolor. These brushes are amazing, you can load the brush and it absorbs and holds the color. With a less expensive brush you will return to the wash more frequently, making more strokes. Less expensive brushes have some sable in them and squirrel, and ox hairs. There are soft synthetic brushes used for watercolor and they can serve your purpose until you are ready to move up once you are happy with the progress you have made. The synthetic brushes need to be wash well and reshaped by hand, this will keep their life a lot longer. Round brushes should always come to a point and stored standing up.