HD satellite has a better image for me on when i see a movie that i have on dvd. Also it depends on the print quality used to make the dvd. Sony Channel, MGM HD, HDNET Movies, and sometimes TCM show some nice looking movies. I also use a HDMI cable from the box to the tv.
I've seen instances where the DVD looked better and instances where the hi-def cable signal looked better, although that could simply be a question of print/transfer quality. I remember when a copy of THE SECRET OF CONVICT LAKE (1951) turned up on YouTube and it was vastly superior to the print that aired on the Fox Movie Channel. Go figure...
But, for the most part, when I watch movies on TCM On Demand, they look as good as or better than a DVD. Fox and Encore Western sometimes crop the image of widescreen movies. I started to watch ONCE UPON A TIME IN THE WEST on Encore Western recently and stopped when I realized it was cropped on the sides to about 1.85:1, whereas my DVD copy of the film has the proper aspect ratio.
I suspect the OP's question may not reflect what they actually want to know. By definition, any original HD source will look better than it's SD [DVD] counterpart, all other things being equal. No need to argue that point, but I think the OP is assuming everything broadcast over cable/satellite is "HD" [720P or higher] which is not at all the case. A source is a source. Generally speaking, [I do understand up converting algorithms] it cannot be natively improved upon. In most cases, cable/satellite broadcasts actually degrade whatever source they receive in favor of cramming more channels onto their respective delivery pipes. They exist to serve the masses.
So, my short answer to the OP's basic question is as follows. Given the same source material [a huge unknown for cable/satellite broadcasts] an equivalent physical disc will always provide better quality since physical discs exist to serve the individual.