I'm against propane injection in a diesel.
Adding propane is just substituting one fuel for another. Besides, as a gaseous fuel, it predetonates in the cylinder while compressing as soon as temperature is reached, because it's unregulated.
Nitrous is nice that way, its an oxidizing agent. Not a fuel source. It cools the charge, adds oxygen molecules. Forces thermal efficiency to go up.
Water meth, is the best compromise, IMO. Although methanol is a fuel, water is the oxidizer. Cheaper than both other options too usually.