What's the difference between double jump and power jump in Cyberpunk 2077?

For the most part, this is going to come down to taste, and how the separate mechanics feel to you in play. The Power Jump does allow for slightly more altitude and distance, but in terms of accessing areas, I haven't found anything where I'm meaningfully cut off from access yet. Meanwhile, the double jump does offer a bit more control and reactivity - you can time it well to, for example, minimize fall damage, and use it to move around corners a bit more readily as well.

If you're interested in exploring the games... glitchier side, there are some wall-hack style shenanigans and otherwise inaccessible areas that double jump will allow you to reach, but power jump won't, by exploiting the abilities interaction with time-slow cyberware like the Kerenzikov.

Basically, it all comes down to which method you feel more comfortable using. Alternately, money is really pretty plentiful in CP77; buy one, try it out for a while, when you've got some spare cash, buy and try the other, and know you can always switch back and forth at your friendly neighborhood ripperdoc at no further cost.