As purely a data storage format, obviously Blu-ray has the potential to store more data than HD DVD.
However, as someone who has been following the whole BD vs. HD DVD consumer video format war, and as someone who has bought an HD DVD player (and, until recently, had a BD video player on order), at this (albeit early) stage of the game, HD DVD is the superior video format.
HD DVD has 30gb dual layer discs available (almost all the latest video releases on HD DVD are 30gb dual layer.) There are many more titles available for HD DVD right now (probably because it's been out longer and the discs themselves are easier to manufacture.) HD DVD uses a more efficient codec (Microsoft's VC-1, which is akin to H.264, in that it's much much more efficient than MPEG-2.) HD DVD titles have either Dolby Digital Plus (a higher bit-rate multichannel audio codec) and Dolby TruHD (a lossless multichannel audio codec).
BD only has 25gb single layer discs available now. Apparently the 50gb dual layer discs are hard to manufacture and the yields are not ready for prime time. No BD retail video discs are above 25gb single layer. No timetable for 50gb discs has been announced. The video is MPEG-2, meaning it takes up more space on the disc. And, the most recent BD releases all suffer from more MPEG artifacts than any HD DVD releases. BD audio is either standard Dolby Digital or space consuming uncompressed PCM audio (which sucks up even more disc space, leaving even less for video.)
The current Samsung BD player actually has the same (Broadcom) chip that the current Toshiba HD DVD player has in terms of outputing video... and it only outputs 1080i. The Samsung player tacks on another (Faroudja) chip to deinterlace it, so it outputs 1080p (so BD can say "we output 1080p!"), except, that chip apparently stinks and makes the picture somewhat soft. In reality, any HDTV worth its salt can easily deinterlace 1080i signals, so the whole "we output 1080p" is a false advantage anyway. Both BD and HD DVD discs store the video as 1080p, by the way.
So, what you have, on the video front, BD has a smaller capacity disk with less efficient video and audio codecs (that look and sound worse). And it is TWICE the price ($500 vs. $1000). And has less titles. And is late.
If you read any reports on BD video quality vs. HD DVD video quality on boards like AVSforum.com, HD DVD beats BD hands down.
Who knows how this video format war will shake out, but Blu-ray is way behind right now.
-Terry