Dawn of War 2 has one of the best single player campaigns in an RTS hands down period this is undisputed, CR is just even more awesome when it comes to single player.
Okay here is the deal with multiplayer, Company of Heroes is the true sequel to the first Dawn of War games, if you like base building, large battles and such, go with Company of Heroes (also note, CoH is in no way fun or balanced in multiplayer and you will be rushed in the first minute).
Dawn of War 2 tried to more emulate the table top and it is based around mass, micro management of usually 3 or 4 squads trying to hold usually 2 or 3 objective points, it is actually really great if you enjoy micromanagement and prefer aggressiveness ahead of turtling and base building.
They are two very different style of games:
Dawn of War, Starcraft 2, CoH - Macromanagement, mass building and constant rushing and pushing out units.
Dawn of War 2, Warcraft 3 - Micromanagement, based around total control of a very small number of highly customizable units and using their abilities to the fullest.