Physical Media
Everyone here keeps focusing on the physical media as this is what is being pushed and in that respect Blueray is the superior format.
However lets actually take a look at whats on the disc.
Blueray: MPEG2 - Lower quality, more artefacts, more space. However it will be easier to get lower in price as the hardware for the drive itself falls, there are plenty of MPEG2 decoders around and they cheap to manufacture. Due to Bluerays capacity they can deal with some of the quality problems via bitrate alone.
HD-DVD: MPEG4 - Higher quality (more modern compression algorithms), less artefacts, less space. However it will be harder to keep dropping the price on these due to the fact that MPEG4 decoding uses a massive amount of processing power, as an example a dual core 64bit processor with the best software decoding available today still takes around 60% of each cpu.
I Own: HD-DVD because I got it practically free when I swapped the XBOX 360 accessory pack for a HD-DVD addon and 5 HD-DVD's. Not that I evangelized HD-DVD at all and if Blueray starts to take a major major lead I will avail myself of a Blueray drive (which ever is cheaper a PS3 or standalone)