How much difference does the fire rate of fast moves make?

I had a debate with a community member recently about which fast move to use with gigantamax Kingler against moltres. I thought that bubble is the obvious answer as it has same type attack bonus and it's super effective.

He argued that metal claw is better even though it misses out on STAB and is resisted damge against moltres. His theory is because it has a shorter cool down between attacks (literally metal claw has twice the rate of fire that bubble does) it therefore will get to the dynamax phase faster and because it's gmax move is locked to a water type attack it will be doing more damage overall with just STAB, super effective damage during the dynamax phase.

Does the cool down on fast attacks effect the generation of dynamax energy that much and is it significant enough to be worth sacrificing the damage done during the normal phase?

Bubble or metal claw for gigantamax Kingler?