Personally, depending on the area, I will always recommend using an old fashioned TV antenna to pull in the best HD picture available. To find out what stations are available free in your area enter your address at TV Fool . In general greens, whites, and even red charted stations will come in depending on the antenna used. All you need to receive them free of charge is an antenna and a digital TV or converter box. DVRs are also available for free wireless digital television. Free over the air viewing can be supplemented with free web video, Netflix, Redbox, or evened borrowed DVDs from the public library for a lot less than Comcast or Dish Network. In a short time you will have saved enough money to buy a giant screen HDTV to enjoy the best available HD picture.
I've used both Comcast and Dish Network DVRs over the past 2 years. Both offer about the same value in terms of usability and features. I'd say in terms of pricing, Dish Network is more competitive with most of their tiered plans. Anyway, I'm not sure if you're asking if DVR or OnDemand is better, but I'd defintely go with a DVR if you're going to subscribe to either service.
I like the 30 second skip on the Dish DVR. I don't know if Comcast has updated their DVRs, but the old ones made you have to fast forward through commercials, which took more time and was less user friendly.