90 volts across 50 ohms would be 162 watts and 600 volts across 50 ohms would be 7200 watts. Into a higher impedance the voltage will go higher, so the 90 volt version would be for very low power or receive only and the 600 volt version might be used for full legal limit power. You never want the gas tube to fire when your transmitting but you want it to be useful to shunt high voltage from lightning.