Having had both. I noticed that the picture quality with DirecTV is better. I'm not sure of the technical aspects - just what I noticed from watching.
That being said I would take Dish Network any day of the week over DirecTV. Having the same bill every month makes up for any slight difference in video quality.
Also, unrelated - but it speaks volumes - with DirecTV they were airing a commercial for discounted Showtime (I think it was $5/mo for 6 months at the time), I called to add it and after speaking with 3 CSRs and having the issue escalated, I was still unable to add the promotion. The CSR flat out told me that it didn't exist, even though the commercial aired on tv while I was sitting on the phone with them.
Dish Network is currently airing a promotion for Showtime $5/mo for 3 months. I called and the CSR immediately added it and told me that I qualified for other discounted movie channels as well. She even had the ability to setup so that Showtime would automatically end after 3 months and would be removed from my lineup. Super nice and knowledgeable.