High-Def Digest just released their review of both the HD DVD and Blu-ray version of Planet Earth. The video quality and audio quality sections of the review for both are identical, because both discs use the same VC1 encode and DD 5.1 audio (488kbps). According to Hi-Def Digest this is a must owned demo disc for both formats and that other than the normal low light artifacts that video is known for, the video quality is some of the best they have seen for either format. The audio on the other hand leaves something to be desired, but isn't bad. The good news is that neither format showed the type of compression artifacts that we have some to live with from sat or cable and that both versions outshine the broadcast version you may have already seen.
Read HD DVD review
Read Blu-ray review