I'm personally not a big fan of using gas to spray the intercooler. It sounds dumb, maybe risky, and definitely expensive. Spraying nitrous on the intercooler is dumb and a waste. CO2 can do the same job, probably for less, and is more commonly found.
Either way, water probably does almost as well and is far more plentiful and cheaper. Its also safer, and probably easier too..IMO water is the best choice.
Spraying N2O in the intercooler is also an idea, but not as good as a straight up direct port IMO. I personally dont care much for nitrous because it has a whole new set of risks that boosting alone already has. Most of all, its another fuel source to refill in addition to pump gas. I personally would only consider water as a supplemental source, and nothing else.