Directv broadcasts (and records) at 1920x1080 (1080i/1080p) or 1280x720 (720p) depending on the channel.
While it is not downrez'd, the encoders do compress the signal using MPEG4 to broadcast. It is not as bad as the compression that some competitors use known as HD-Lite, at which point they were downrez'ing 1080i to 1440x1080 as well as compresssing.
DirecTV generally keeps 5-6 channels to a HD transponder, while I think Dish does as many as 7 IIRC, though they use different encoders. Either way, I have watched both and felt DirecTV HD PQ is better than Dish HD PQ.