Think of it in terms of the size of each stair and the size of the person.

If each stair is > the size of my leg, each step is plenty of work. Doubling it engages more muscle groups. In this case obviously more energy.

If each stair is < the sole of my shoe, each step is barely any work. Doubling it is negligible, no new muscles are engaged, they just work slightly harder for double the gain.

So, the answer is both, and it depends on the size of the person and the size of the stairs.

Moving the same thing (your body) from the same starting point to the same end point will accomplish the exact same amount of work, and therefore require the same amount of effective energy to accomplish, so it boils down to biomechanical efficiency of which movement more efficiently accomplishes work, and wastes less energy.

That’s going to depend on the individual, as everyone’s limb length and muscles and tendons will affect how efficiently the body maintains forward momentum once in motion, how much energy is wasted on each step, etc., so that the ideal step size is different from person to person, and the ideal speed is different from person to person. The answer will change for the same person with training, too, as the biomechanics of a particular movement tend to get more efficient when that movement is trained over time.

You may be able to estimate the right answer by just doing it both ways and seeing what a smartwatch tells you. The gold standard would be to hook you up to a machine that measures your oxygen consumption, but that wouldn’t be very practical.

Assuming standard 7 3/4in height and 10in tread depth.

Skipping steps is faster. The height and distance energy burn is roughly the same whether you walk up normally, backwards, skipping steps, or crawling (within reason). However your baseline metabolism is constant, so the faster method burns fewer calories.

Let’s say you were walking up stairs for 10 miles. That’s about 40,000 stairs. I would estimate that would take about 10 hours to do. But skipping steps you’d be done in maybe 6 hours. So in either case the stairs burn say 5000 calories. But going slower you have like 1000 calories of baseline metabolic burn, and skipping steps only 600. So skipping steps might be about 7% more efficient overall.