DailyTech has an article about the differences between HD DVD and Blu-ray, as demonstrated by the new 300 movie which came out in HD this week. Overall the HD DVD version outshines the Blu-ray version:
The rights to “300” belong to Warner Home Video – a studio that backs both HD DVD and Blu-ray Disc – but the studio has graced the HD DVD version with several exclusive features that currently can be found nowhere else.
Found only on the HD DVD version is the "Bluescreen Picture-in-Picture Version" of the film. As “300” was shot almost completely in a bluescreen-laden warehouse in Canada, the raw footage differs greatly from the movie’s final look. Viewers are able to directly compare the before and after shots through a picture-in-picture window that can be dynamically enabled or disabled. Running alongside the pre-processed footage is an exclusive commentary track by director Zack Snyder recorded specifically for the special feature.
The bluescreen supplement is not found on the Blu-ray Disc version of the film, but Warner Home Video’s decision to include it only the HD DVD version is unlikely due to any sort of format favoritism. Current mandatory Blu-ray Disc player specifications do not include the feature set to allow for picture-in-picture video.
The Blu-ray Disc Association has mandated that all players of the format released after October 31, 2007 must support BD Java, a programming language for Blu-ray Disc media used mainly to deliver picture-in-picture for in-movie commentary and special features.
The HD DVD equivalent of this enabling feature, called HDi, is already standard on all HD DVD players. Rather than being based on Java, however, HDi is built on Microsoft’s XML standards.
Another feature that points out the feature differences between the two high-definition player specifications is HD DVD’s requirement of being Internet-connectivity ready. Recent firmware updates for HD DVD players, which are obtainable via the web, have enabled “Web Content” features for specific movie titles.
“300” on HD DVD takes advantage of online content by allowing the viewer to browse and purchase movie-related items, such as ringtones and wallpapers, for use on mobile phones – another feature that is exclusive to the movie’s release on the format.