Tell the Cable company that you are thinking of switching, even if you don't have DSL available yet. It's the truth...You are thinking of switching (even though you are not able to yet). Maybe you can speak with a higher up Rep with the Cable company (Customer Retention?) and get them to cut you a deal that they offer to new subscribers on a daily basis.
As far a picture quality. DirecTV's HD service is the only service I've had with HD. To be honest, I never had Comcast television service, only internet.
Compared to my Father-in-laws' Dish HD, I don't see any major difference other than the TV's that the services are on. He has a 50+ inch 6 year old Sony Projection HDTV. I have a 2 year old Toshiba 30" HD.
Although, I have noticed that since I switched to the HD DVR, my Directv image has improved greatly for all channels, but I believe that may be attributed to using Component cables (HDMI had compatability issues with my TV).