[Richard L]

Water injection and alcohol injection requires a different strategy to your tuning package and can be summarized as follows:

Inlet air cooling:
Both does the same job but under a different temperature range due to its freezing point and boiling point (-90C to +70C for methanol) and water between 0C to 100C. You need to inject twice amount of alcohol to one part of water. If you mix the two, you get a very wide operating temperature range and less amount injected in total. Recommended.

In-cylinder cooling:
Assuming you have already running an ideal a/f ratio of 12.5:1 (in general), neither liquid will be needed as extra fuel (alcohol). In this case, you need to consider water injection is better to control your peak combustion temperature spikes - latent heat is two times higher than alcohol. In order to have the same effect, you need a system that will flow twice as much with methanol and four times as much with isopropyl.

latent heat value by mass, injected quantity by volume (...):
Water ... 2256kJ/kg (1)
Methanol ... 1109kJ/kg (x2.53)
Ethanol ... 904kJ/kg (x3.10)
Isopropyl ... 665kJ/kg (x4.24)
(Gasoline) ... 350kJ/kg (x8)

From the above figures, you can see how much more alcohol is required to be onboard and the pumping system has to be capable of flowing much higher than just injecting water.

Octane value and knock resistance:
This is the stage of the comparison where water and alcohol injection parts company. Water does not have octane value or can be regarded as infinite since it will not ignite under the greatest pressure and temperature. Knock prevention works completely different to alcohol. Alcohol is octane rated so if alcohol is used a a fuel additive, so your fuel's octane is directly proportional to the amount of alcohol injected. I don't think I have the required knowledge to tabulate a comparison of the two concept.

My conclusion:
Water injection and alcohol injection is totally different when part of the alcohol is used as a fuel supplement and results should not be compared. It is like comparing a gasoline fuelled engine and alcohol fuelled engine