I was researching HD-DVD and Blu-ray for an online posting, and some of the information I found is pretty interesting. Here's some of what I found, based on information from www.blu-raystats.com and Wikipedia.
HD-DVDs are nearly all released on its higher capacity 30GB format. As of this writing, 51.97% of Blu-Ray discs are released in the lower capacity 25GB BD25 format. Thus the apparent size advantage of Blu-Ray isn't currently being utilized by slightly approximately half of the currently published BR discs. The HD-DVD specification was recently updated to support 17GB per layer, up to three layers (or 51GB) per disc. Blu-Ray officially supports one or two data layers up to 50GB total.
38.11% of Blu-Ray discs are released in the MPEG-2 format. This is the older format that has been blamed for poor video quality on early Blu-ray discs. While the 50GB BD50 discs with MPEG-2 are certainly better than the 25GB BD25 discs, AVC and VC-1 encoded discs offer much better image quality, even comparing these formats in a 25GB format to a 50GB MPEG-2 disc.
Before buying a Blu-Ray disc, check the www.blu-raystats.com site to make sure it isn't a 25GB MPEG-2 disc, as these are the ones with image quality problems. A 50GB MPEG-2 disc will look fine for movies of average (or shorter) length as long as there isn't much bonus material on the disc. I am not aware of any general image quality issues with HD-DVD discs.
Other format differences: HD-DVD supports Managed Copy which allows you to copy your movies to a home theater PC, though I'm not aware of any implementations of this just yet. Blu-ray does not have any such capability.
HD-DVD has more interactivity features than the BD 1.0 specification, though BD 1.1 attempts to address this. In the real world, this means that HD-DVD discs currently tend to offer more visually appealing menus and more disc features. This is likely to change slowly with the adoption of the BD 1.1 and 2.0 specification.
Both formats support the same video formats. Audio format capabilities are effectively about the same, with some variation on which competing formats are utilized for different levels of compression. Both support uncompressed audio in at least 7.1 channels.
Both formats support full 1080p resolution at regular TV refresh rates as well as 24p, with effectively all movies being released in that format. The implementation on the discs is slightly different, but the data is the same. Some HD-DVD players only output up to 1080i, but many 1080p TVs are fully able to reproduce the original 1080p signal for display. To take advantage of a smooth 24p-based cadence, players in either format must be connected to a 72 or 120Hz television via HDMI. Connecting to any other type of television or using any other type of connection will result in 3:2 pull-down being added to output video at 60 Hz.
Movie studio support for the two formats is pretty much a toss-up, with current offerings in both camps having almost exactly equal numbers of titles available.
HD-DVD does not have any region coding requirement, so you are always free to import discs from overseas and play them on any player. Blu-Ray uses three region codes (A,B,C) to make sure that encoded discs are not played in regions other than those they are intended for, similar to the way that DVDs are region coded now. This in some cases is a disdvantage for HD-DVD, as sometimes a disc release for a short time is delayed while the movie continues to show in theaters elsewhere in the world.
In several cases if you are not able to obtain a movie title on the format of choice in your home country, it might be available on the other format elsewhere. If you have elected to use Blu-ray as your format of choice you need to make sure that the disc you are purchasing from overseas will play in your region, however.
Some/many HD-DVD discs are available in a "combo" format (usually at a higher cost) which contains the high definition version of a disc on one side, with a standard DVD version available on the opposite side for playback in regular DVD equipment. Blu-ray does not offer a similar capability.
Discs prices are pretty similar between the two formats, with both being significantly more expensive than DVDs.