I already replied to your posting in the other thread, but here it is again.
As for Dish Network and DirecTV, the HDTV is of comparable quality (no difference I can tell). On most standard channels (there are exceptions), DirecTV has better quality than Dish Network. Last year, DirecTV went through a period where their picture quality was pretty bad, but some months ago, they used the bandwidth regained by off-loading locals [to a new spotbeam satellite] to improve picture quality. Dish Network could do the same, but they have not done so yet.
As for digital cable, that depends on your particular cable network. The quality of digital cable varies widely by cable company and location. Some digital cable networks look better than DirecTV. Many look worse. Some look much worse.
For the record, I currently subscribe to both Dish and DirecTV, and will be adding Comcast soon (for its upcoming HDTV regional sports network with all Orioles, Wizards, and Capitals home games in HDTV).