Dawn of War is more of the 'traditional' RTS mechanics with some base building, specific missions and scenarios following a story etc. I think?
Whereas this I would expect to follow the traditional TW mechanics - a massive 'galaxy map' where you're moving fleets and armies about between planets (we get a brief glimpse of this at the end), with large scale pitched battles of those forces wherever they conflict.
It's a little disappointing that they both launch with Space Marines and Orks as 2 of their 4 factions but I guess on 'mass appeal' basis, those are the two big hitters from 40K.