Regular speedball ink is more like acrylic, so if you don't frequently wash your brushes even in the middle of a piece, it can gum them up really quickly. Sumi ink is more like watercolor, in that if you wash your brush after you work for a while the pigment all washes out without causing any permanent damage to the bristles (I also prefer how it thins out for washes more than regular speedball). I scan all my inkwork into photoshop anyways, to color or prepare it for printing, so being as super waterproof as speedball ink isn't as important to me.