Depends on wheter you live in the US or not
To me the answer i give is pretty simple:
Live in the US and you can use either format, it does not matter much. Quality is about the same on both formats (although i have my doubts about BluRay using MPG2..).
If you are NOT living in the US or Canada you are better of with HD-DVD:
No region encoding (btw: regions are shifted now, Japan is no longer Region 2.. why? because Americans could not import Anime dvd's.. so Hollywood decided to take Region 2 from the Europeans and add it to Region 1, there are now 3 regions for BluRay in total, HD-DVD has no region coding).
Don't let people fool ya saying BluRay is superior, it is a medium with higher storage capabillities (25-50 opposed to 15-30 and soon 3 layer 45GB for HD-DVD).
A decent 2 hour Full HD movie with mutiple audio is about 20GB with the new MPG4 AVC or VC1 or H.264 codec so they fit on either format.
They both support the same codecs for audio/video, support the same resolutions, HD-DVD is cheaper to produce as it is an upgrade to dvd technology and production lines need not be altered much (BluRay needs new production lines but who cares: Sony does not allow any BluRay being produced outside their own factories).
Value documentaries and old movies? HD-DVD does NOT have mandatory encryption, so productions for the public domain do not need to buy a (for BluRay mandatory) 5000 usd encryption key per title.
BlueRay uses too much drm crap.. is this important? Well, let's say i'd like to see what happens on a scratched disc when the stream gets interupted and the BD+ virtual machine kicks in to grace you with a 'unable to play this pirated disc' message ;-)
If your new HD capable mediacenter in the near future plays this content 75% of the decoding capacity is used for the drm and 25% is used to actually decode the stream. That's why we will need better cpu's..
I will never buy Sony again and since i am a European my vote is for region free HD-DVD (WTO, free trade anyone?)