I found this interesting. This is from the dolby site talking about DD PLUS -
Features
Multichannel sound with discrete channel output.
Channel and program extensions can carry multichannel audio programs of up to 7.1 channels* and support multiple programs in a single encoded bitstream.
Outputs a Dolby Digital bitstream for playback on existing Dolby Digital systems.
Supports data rates as high as 6 Mbps.
Bit rate performance of at least 3 Mbps on HD DVD and up to 1.7 Mbps on Blu-ray Disc.
Accurately reproduces what the director and producer intended.
So the bit rate of HD_DVD is much better than BD. Another reason to go HD_DVD. With DOLBY behind HD_DVD over BD (not officially) this makes HD_DVD look even better.