In my opinion, they are both important and it is very hard to say that one is more important than the other.
Economically, power is very important. You need power in order to have a strong economy and you need a strong economy in order to allow people to live good lives. Having people live good, comfortable lives is really important. We in the rich world don't really understand how important it is because we take it for granted.
On the other hand, if we degrade the environment too much, we also degrade people's ability to live comfortable lives. Global warming, for example, may destroy countries like the Marshall Islands that are made up only of atolls.
From my perspective, this is all about giving people better lives and we have to have both power and a decent environment to make sure that as many people as possible can have decent lives.