A quad layer BD has 100GB and 50GB BD dual layer discs are here now, ready for use, viewing, and are recordable. HD DVD tripple layer 51GB (1 GB more than dual layer BD) for read only no recording or RW, three layers to beat a dual layer by 1GB and it hasn't been accepted as a part of the HD DVD standard (yet), so it won't work with the current HD DVD set-top box specification. Things could have changed though.
The biggest feature I like about BD is its scratch resistance, as I tend to drop discs on concrete.
Clean the thing with steel whool and see what happens(check out YouTube), can't do that with an HDDVD. Then there's BD-J...
Dragon Lair anyone? Blu-Ray also has higher AC-3 audio bit rates, i.e. better surround sound capability but the most compelling reason why i hope Blu-Ray wins is.... Microsoft. As a Mac Zealot, it'll be cold day in Hell before I let Bill or Balmer into my livingroom.
Also, the movie industry is real keen on protecting their intellectual property. AACS, the only encrytion scheme available for HDDVD has been cracked; BDs have BD+, in addition to AACS, which hasn't been and even if it were, BD+ is dynamic and can be changed... studios like this security advantage/option Blu-Ray has over HDDVD.
Movie Titles: Blu-Ray has more titles coming out for release from every major studio and independents, except for Universal.
my 2¢