The map and it's buffs are a little more digestable and still interesting, story is a bit better, and the adjacent armies in DC campaign also just feel better. IMO Dark Crusade's campaign is better than Soulstorm's. If Skirmish is what interests you then the others can say more, as I've no interest in Skirmish myself. If you're in it for the campaigns and a decent introduction into the Warhammer 40,000 universe, I would recommend starting with the original. They are both very nice but most maps feel like simple Skirmish maps, there is only more story involved when attacking the Stronghold provinces, also the difficulty is higher than in the original.

Both campaigns also introduce a novelty, the Global Map where you get to choose which province to attack like you do in a game of Risk during your turn, plus some other cool features. On the plus side you get to choose between 7 races in Dark Crusade (Space Marines, Orks, Chaos Marines, Eldar, Imperial Guard, Tau and Necrons) and 9 in Soulstorm (+ Sisters of Battle and Dark Eldar). The story in Dark Crusade and Soulstorm's campaigns is a bit more barebones, they basically just give you enough reasons to bash the other races. Winter Assault's campaign is interesting, but it's very unbalanced and it may become frustrating-here you can choose between the Order (Imperial Guard, Eldar) and the Chaos (Orks, Chaos Marines) factions. The original has the best campaign, very cinematic and balanced, it is a joy to play however it focuses on a single faction, the Space Marines.

Dawn of war soulstorm campaign mods