Plate system vs. Nozzle system.
There are pros and cons to both styles of systems.
A plate system is easier to plumb, costs less, but will hit harder. While a nozzle system will be easier to fine tune, run more precisely, and doesn't hit the chassis as hard.
"When should I go from a plate system to a nozzle system?" The main reason would be if you were having distribution problems. Certain intake manifolds will inherently have distribution problems (rich, and lean cylinders) and running a plate nitrous system will make this worse. The next reason would be to dependent on your available traction. You can run more nitrous through a nozzle system than a plate system since it will not shock the chassis, and tires as much giving you better performance in certain applications. Lastly it would be a matter of rules, or the look that you want.