IF there is a differnce in PQ, it may boil down to "philosophies" or subjective decisions as to which data will be reduced to allow for other data that, as a compromise to the limited bandwidth for both DirecTV and Dish, results in the best PQ possible according to WHOever makes that judgement, and that is incredibly subjective.
For example, we are all a bit guilt of speaking in imprecise terms when we say the picture has too much compression, as if the level of compression is a single gross, macro application compressing all data parts of a picture and SOUND equally (if we want more data for the best possible DD 5.1, then the PQ is going to have to decrease, even if harldy noticable in order to get what we want, which is why MVPD's use a lower bit rate for audio and often have inferior 5.1 sound to, say a Blu-ray's 5.1 stream).
There are data components to a moving picuture that any MVPD can choose compress to different degrees, especially with MPEG4, such as luminance, chromanance data and more. Dish seems to be crushing the luminance data as of late on HD, including the premium movie services, more than I have ever observed (while the VOD of the same movie had more luminance data and was, overall, a touch superior in PQ), and it can look ugly at times, but a subjective decision has been made that it is better to have higher banding in a few dark scenes than suffer, say, inferior color data or other compression artifacts in, say, high action scenes. Maybe DirecTV doesn't need to compress as much of the data as Dish and that is why some consider their PQ better, or is it that DirecTV has made different choices of what to compress to have a subjectively more aesthetic apperance than Dish.
It can be something like the disagreement if film grain is unavoidable if one truly wants the highest detail in a picture, or is the grain noise, an unaesthetic distraction to what is supposed to be the immersive and beauty of the picture quality? One person will believe accepting the compromise of film grain in order to have the highest detail is preferred, while others will believe the reduction of the detail--or a "softer" picture--is a worthy compromise to eliminate the distracting, unaesthetic "noise" of film grain. Some people LOVE film grain, while others hate it. I am in the later camp, but I know more than a few in the former camp.
While lack of bandwidth seems to be more of a problem at Dish, I really do think it is more a matter of subjective philosophies as to what results in a preferred PQ by both the MVPD and the viewer. It sure seems as if CR didn't really set out to answer the question to their own supposed high standards. A survey of PQ, especially when Dish and DirecTV are not grossly different in PQ, seems a bit lazy of CR. I know people who think SD and foggy images are GREAT TV PQ and don't see much difference with HD compared to SD. So, what are we to expect from a lot of folks who may have no standards of what PQ is answering a survey. It would be far more valuable to the consumer if CR would approach this interesting subject that may be the primary or near the primariy factor in choosing Dish or DirecTV, in the way it does its other product review: with some degress of objective data and some subjective impressions.