Nintendo Power’s guide is far inferior to the Prima and BradyGames counterparts as it is much shorter and far less detailed. One of its best features is that it does have some more realistic looking dungeon maps that are very simple to follow. It is also the official released English guide from Nintendo, and thus can be considered official.
There is very little reason to purchase the guide unless you are strictly a collector. The BradyGames and Prima Games are just far superior. Its only saving grace, is the official Nintendo Power guide is incredibly cheap, with used copies selling for as low as $2 at Amazon.