If I owned both BD and HD-DVD, it would mean that I couldn't wait for HD-DVD to die, and was ok with my HD media been scattered on 2 disparate formats.
If that was the case, how is it a question? the HD-DVD version, with more features is the obvious answer.
... but because I do care about the longevity of the format; and that I dislike having a collection of movies that I need to play on a specialised or an outdated player, I don't own a HD-DVD.
As an extension to that; if you gave me a HD-DVD player now, I'd still buy the BD version of 300, even though, I like the IME extra content, because I know, in the future, I'll want to play 300 on any player, not just the one HD-DVD player I have.