More powerful water guns have higher outputs compared against less powerful water guns, at the same nozzle diameter. Therefore some sort of output to area ratio would approximate power. Flow per unit area comes out to units of m/s, which is velocity.
Therefore, for the same nozzle diameter, the gun with the higher velocity is more powerful. Seems simple if you think about it, but no one has compared water guns like that yet.
I'm posting this mainly so I don't forget it.
