I've owned and fired both quite a lot, and the basic difference is heat dissipation by the water jacket/steam condensing can. The 1917 can fire all day long, or until you have to change out the barrel or run out of water. Other liquids can be substituted but one in particular which GIs in the field had to resort to makes the gun quite, well, smelly when hot.
The 1919 in contrast is designed for burst fire, and I can assure you sustained fire WILL overheat the gun, causing all manner of problems, from tightening headspace to cook-offs in the chamber.