space is irrelevant since a lot of BD come in two disk (pirates...) varieties when the need is not there, just to appear as better value.
two words: finalized specifications
every HD DVD is region free (a huge plus; cheaper and more releases overseas)
every HD DVD will work 100% in every HD DVD player (not the case with blu-ray, thank you profiles...)
every HD DVD has the in film pop-up menu as standard, on BD this is optional
every HD DVD goes straight to the menu or film with out trailers and crap,
not the case with BD. 3:10 to Yuma which has 14 minutes of trailers which must be fast forwarded to get to the damn movie.
Transformers in HD :P