Currently HD DVD is only available in 30GB discs while Blu-ray already has dual-layer 50GB discs, Sony is planning to take advantage of this by using the extra 20GB to deliver higher quality:
Sony realizes this distinct advantage, and is committing to rolling out 80 percent of all its forthcoming Blu-ray titles as 50GB discs, according to Video Business.
When transferring a film onto Blu-ray, compression engineers may utilize the extra 20GB to attain higher bit rate video and to accommodate lossless audio streams. While many dual-format releases of late, such as The Departed, feature identical video streams encoded with the same codec, engineers recognize that the added space afforded by Blu-ray can be spent to improve quality.
The encoders responsible for Nine Inch Nails: Beside You In Time, released on both HD DVD and Blu-ray acknowledges the difference. On the difference between the HD DVD and Blu-ray versions of the concert, a FAQ document on the NIN Web site reads, “Technically speaking, the video quality of the Blu-ray version has a slight edge over the HD DVD: It was encoded at a slightly higher bit rate due to the Blu-ray spec's higher bandwidth capabilities for encoded video streams. However, this difference is nominal and would only be noticeable by a pair of well-trained eyes on an extremely expensive professional 1080P monitor.”