Hill climbing is complex, and hard to break down into simple numbers.

If you look at raw horsepower to go up the hill, Bike 1 takes more horsepower to go up the hill than Bike 2 at a slower pace.

But, say you alter your cadence sufficient to hit 5 MPH on both bikes. Then you're using the exact same amount of power.

Yep throw it into say 50/25 gearing, and slow down your cadence a bit to hit 5 MPH... still the same amount of power (although perhaps more stressful on the body with standing, pulling up on the bars, pedals, etc, on some hills).

The other factor related to effort is duration. So, I can do pretty well with any effort sustained less than say 1 minute. Beyond that minute, I start slowing down rapidly.

So, I may well be better off climbing a short hill at 10 MPH in 1 minute than climbing the same hill at 5 MPH in 2 minutes.

Extend that to a longer hill...
Say you can climb the hill at 7.5 MPH for 1/2 hour, vs 5 MPH for 45 minutes. You're over the hill and coasting down in the first example long before you get to the top in the second example. Yes, a lot more work for that extra 2.5 MPH, but it is over quicker.

Beyond gearing, you can look at things such as crank length. So, say you're comparing cranks from 165/170/175/180mm. In a sense, your cadence is related to the circumference of the crank stroke (2*pi*radius). So, longer cranks give more leverage, but slower cadence. No free power. But, it may also impact your overall gearing choice.