For me, DOI is just SCD methadone. SCD has a much better atmosphere, the set is smaller so it's more involving to watch, there's more of a cameraderie backstage between the couples, the pros don't have to play second fiddle to T&D and there's more of an opportunity for them to stand out and launch their own career. We see a wider (and better) range of choreography because more individuals are involved and can bring their best ideas. Because the choreography is done by the pros, when the pro line-up changes, the choreographic style changes too so the show doesn't get stale. Using different dance styles each week gives more structure to the routines and the audience goes in with an expectation of what the dance should look like. The judging on both shows is not great, but on SCD you get the impression that criticism isn't as personal and that there's a good vibe backstage. The celebs don't need to put themselves at as much risk of injury or accident than on DOI.
It's just... everything about SCD is better. There's the makings of a great show in DOI, but ITV always go for a certain production style that seems to remove all atmosphere. It all just seems a bit soulless and nasty - I feel like DOI chews them up and spits them out whereas even the worst celebs on SCD seem to get a kick out of taking part.