Well I'll give it to Dark Crusade on story content even if there isn't that much of a structure, because in Retribution the story is repeated for all factions and it feels real cheap.
But they are not at all the same when it comes to gameplay, Dark Crusade is based on the DoW1 system where you do all the resource, troop and base building with the potential to build full on huge armies. While Retribution uses the new DoW2 system where only 4 small squads are ever under your control and no base or resource stuff going on, but the fights do get more detailed and personal.
Retribution also has a basic RPG system going on with XP, ability unlocks and loot if that is something you go for.