Both. Training power is more or less the same as training for speed. There are differences, however. When I train for speed, I hold an aero position and pedal at a high cadence, much higher than I use for climbing or ordinary cruising on the flat. That's a big difference. It uses different muscles and slightly different pedaling mechanics. One doesn't need to motorpace. One can simulate motorpacing by doing as describing out in the wind. It's not exactly the same thing because speeds are much lower, but if you concentrate on maintaining that high cadence I think it accomplishes the goal of putting out power in an aero position on a road bike.

If you're talking about training power as training for short bursts of peak power, that's sprint training and a whole 'nother subject.

If you're contrasting sprint training with climbing training, no, you really can't have two such divergent specialties. You can switch specialties, but you can't train them both and excel in both because you'll lose to the specialist. That said, there are exceptional individuals who do both well, but they're rare.

And as we see every year, those who train to excel in both TT and climbing do both well, because both depend on threshold power. But they do train both of those specialties consistently or they don't do well.