Blu-Ray.
1) More storage. The 45GB HD discs don't exist in production yet. And remember that while HD has three-layer discs in the lab, BD has eight-layer discs in their lab. The fact remains that a BD layer can hold more than half again what an HD layer can. 25GB/layer instead of HD's 15GB/layer.
2) Higher bitrate for video.. 54Mb/sec over HD's 36Mb/sec. That means higher quality. It'll work hand-in-hand with the higher capacity, to maintain a constant higher bitrate for movies.
3) More Hollywood support. Blu-Ray has pretty much everybody except Universal (for now, Universal will probably follow Paramount and WB and support both). HD-DVD has only three major studios, and two of those are also supporting BD.
4) The Playstation 3. People keep saying this isn't a big deal, but I look at it and I see millions of Blu-Ray players in people's homes practically by default. Like it or not, that's a HUGE advantage for BD. You know damn well that Sony's going to hype the hell out of it.. BD movies will have big stickers on them saying "Playable in PS3!!" so all those millions of households will know that they don't have to buy a separate player to watch them. Maybe it won't make a difference for people who aren't buying a PS3, but I am buying one. And I'd much rather get movies I can watch on a device that I already own, rather than spending another $500 or so on something else. Especially since I'm running out of connections on my reciever for digital audio.
gottagame - In addition to the storage thing I mentioned in point 1, you should remember that BD has a new type of protective layer than HD and DVD are using. Even though the surface is thinner, it will actually be more resistant to scratching than current DVDs are. And I, for one, take care of my games.
Oh, and don't forget the media... I seem to recall the latest report said something to the effect of "HD-DVD's Days are Numbered". The public reads those things, you know. While we may know more than the average consumer in regards to this sort of thing, they're still forming their own opinions based on what's in various newspapers and magazines.. and they're all saying the same thing.. that BD is superior to HD in every single category.
I honestly can't understand how anyone, especially on a forum like this one, could possibly support the lesser format.