I am a self-proclaimed pixel-peeper. I can see subtle differences in picture quality, find macro blocking in the background distracting, and can tell if colors are quashed or good. DirecTV's HD is the best. I just switched to Dish Network and I can easily see the difference. Dish's national HD is very good, certainly watchable, but not as sharp as DirecTV's. Dish's HD locals are terrible for me - perhaps because I live in a small market north of 150th. To save bandwidth, Dish cuts the horizontal resolution to 1440 from 1980, which is at least part of the reason DirecTV's HD is better. If you have a 720p native set, you wouldn't be able to see the difference.
However, DirecTV's SD is unbearably bad. Dish's is very watchable. If a channel you really love is only in SD on DirecTV, you will be very upset at how bad the picture looks. Dish has more HD channels in their basic lineup.
Many people have problems with DirecTV HD DVRs being slow or unresponsive. Mine was, and that was the primary reason I switched. Even as a new customer, you may get a pokey DVR. Dish's DVRs are fast, sometimes jarringly so. If you haven't already, read the forums about DVR performance issues and decide if you can live with a dog if you get one. My HR22-100 had great pictures. That's about all that's good that I can say about it.