But "better" depends on what question is being asked. Power output is very, very useful because it's so closely tied to performance. But it's not great at answering the question, "Why?" E.g. if my power for an hour increases by 2% after some period of training I don't know whether it's because I'm more efficient, have higher aerobic capacity, higher anaerobic capacity, etc. Answering those questions can be useful. If I had to choose just one, I'd pick power. But I don't think it's exclusive of the other metrics. If there was some non-invasive watch that could measure my lactate, that'd be nice to add to the mix.