A siesta is much more in keeping with hotter countries where at the height of the midday sun it’s inadvisable to be doing anything. From an evolutionary point of view on the plains of Africa we would have found a shady tree or a cave and conserved our resources.
So in warm countries a siesta is a sensible adaption to the climate. If you’re sitting down doing nothing you might as well have a sleep. A power nap? Well, if you’re sleepy during the day then what your body needs is sleep, it doesn’t need stimulation, so a power nap is going to be much more effective in boosting your performance than coffee would be – just a 20-minute nap.
However, the question would then be: Why are you sleepy during the day? Power naps are good if you’re burning the candle at both ends. If you are finding it difficult to concentrate and focus on what you’re doing it’s far more sensible to have a 20-minute nap than to have two strong cups of coffee.
But if you had enough sleep at night you should, to all intents and purposes, be able to get through the day without needing a nap.