well neither injection is plug and play, they both have to be tuned to your applicaiton
with nitrous injection (for cooling) you gernally need to retard your timing enough to avoid pre-ignition nitrous is more of a power adder and not so much safety buffer.
while water injector is a safety buffer/ water injection is a octane Improver also.
when you run water injection with methanol and tuned properly it works as both power adder and safety buffer.
both do need to be tuned tho,esp to gain the most safety factor be it using nitrous or water/meth
do also realize that there are safety factors when you do run water injection, like the fact that when you have your car tuned in boost for water injection and you happen to run out of water, this will blow your motor/damage your motor/ you have to consistantly check the nozzle to make sure they are functioning properly. as your motor is relying on it.
i donno there are many factors when you get into power additives