Hd-dvd is a standard promoted mainly by Microsoft, blu-ray is a standard promoted mainly by Sony. Blu-ray is technically superior, but that doesn't mean it will win the "format war"(just like betamax was superior, but lost to VHS). Blu-ray seems to be winning at the moment; they're selling more players and videos, and blockbuster recently announced they would only rent blu-ray disks(and DVDs, but not hd-dvds). If you don't have an HD TV, these are mostly useless to you. If you do, I would wait to see which format wins. If you absolutely need to buy a player for one right now, go with blu-ray.
HD-DVD holds 15GB(about 20 low-quality movies), Blu-ray holds 25GB(about 35). Each can have multiple layers, which in the end could hold every file on your computer.
At the moment HD-DVD players are around $400, blu-ray players are around $600.
A *blank* HD-DVD disk is around $9/each, blu-ray at $11/each.
Movie disks on each format are around the same price, about 2x more than regular DVDs.
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