Comcast is going on the concept that they have more HD "Content" because they have On Demand offerings that are in HD, but even that is questionable, since it depends on how you "count" an HD offering. For example, my Dad has Comcast with all the movie channels, which includes like Starz HD On Demand, but he doesn't get all the Starz channels in HD that D* offers. So which is technically more HD? With On Demand, you have more HD programming available at any given time, but those multiple channels will probably give you more HD movies than what is just offered on the Starz On Demand service.
As far as HD Channels, D* wins hands down. As I said, my dad has Comcast, and while his HD offerings have increased in the past 6 months like D*, he still doesn't get FX HD, ESPNEWS HD, VH1 HD, Spike HD, various RSNs, etc.