In my opinion, this is a better way to do Coronation Street spin-off shows, what is happening in the main show is usually reflected in the spin-off an vice-versa, therefore it furthers the main plot, an example of this would be Gary's Army Diaries.
With a DVD spin-off, the storyline usually has light ties with the main storyline of the show, but for the most part it is a totally seperate story, and usually ends up feeling pretty disconnected from the main show. There is also the cost, it it worth spending up to £20 for a spin-off that you probably wont watch again?
Which do you prefer?