I've never heard of water/methanol reaping the huge fuel economy benefits many claim with propane, but it is my preferred combustion enhancer. Propane can be expensive, and it can raise EGTs. The flash point is far lower than diesel, making it more dangerous.
With water injection, depending on ambient air temperatures, you can use only straight water and still find gains in timing, boost, power, and reduced EGTs. You will also lower NOx which is an environmental incentive. A water kit won't have the constant expense propane refills will. It does require a bit of tuning as has already been mentioned. Also, it can grow with you if need be by using larger injector nozzles.