You are likely comparing different masterings, and consequently, you may prefer one over the other without the format being any better at all. Several of the video and audio codecs are the same for both:
https://en.wikipedia.org/wiki/Comparison_of_high_definition_optical_disc_formats
However, BD allows for greater data transfer per second, so if an HD DVD looks better than a particular BD, it will be due to different mastering. BD is the better format.
For some reason, there is a problem with my link (though it is named properly, it links to the wrong site). Here it is again:
https://en.wikipedia.org/wiki/Comparison_of_high_definition_optical_disc_formats
If one looks at the codecs, one can see that both disc formats support both lossy and lossless audio. If a studio chose to put a lossy soundtrack on a BD, but a lossless one on the HD DVD, then the HD DVD may sound better even if made from the same mastering and only converted to the respective codecs. That, however, is not the fault of the format, but a matter of what choices a studio makes in producing the discs. Both BD and HD DVD are capable of a great picture and great sound. But still, BD allows for greater data transfer per second, as well as having a greater data capacity, so it is the better format. But the lack of region coding on HD DVD is better, as it means that one can view any HD DVD on any HD DVD player, unlike BDs that have been region encoded.
