The way I categorize the differences between them is: DW is almost entirely inward focused. You try to optimize your economic machine and arrange your fleets and refueling stations to protect it. There's simply not much you can do that doesn't involve managing the internal workings of your empire. Sure you can go out and fight wars, but that's simply not interesting. In either game. More tech, more money here means you win the battle, less tech, less money here means you lose. There are little things you can do in both of them but there's no particularly interesting decision making. 4x games are always like that, war is the arena where you find out which side did well at the actually interesting parts of the game.
GC 2 is more of a realpolitik thing with a focus on deciding your strategy in advance and then reacting to unexpected changes. You have a lot more power to gear your empire to being military based, research based, trade/econ based, etc. Almost all of that power in DW ends the moment the game starts because its all in the racial/governmental modifiers and 90% of the tech in DW is ship/station improvements. In GC you generally know what victory conditions and techs you're going after. You also have the power to prepare in advance to make peace with certain races (through trade, picking your alignment, treaties) and to make war with others (by researching defenses against their specific weapons). Once you figure out your neighbors you usually start developing a more concrete plan to deal with each of them one way or the other.
Where GC 2 gets really fun is that the other races are all making these decisions too, and if you're playing on the right difficulty things will never go entirely how you planned them to. Its hard to describe without launching into a review. One game I peacefully built a little alliance for myself out of half the galaxy, only to have the other half declare war on me, suddenly dragging every single race into a huge galactic war. In another, a race on the other side of the galaxy became ungodly powerful and I had my originally peaceful race conquer Earth and all the human colonies soley because I felt that was the only way to compete. The best example I can think of is an AAR where the guy inexplicably survived being at war with the most powerful race, the Dregin, for the whole game. He thought it was because of his desperate defense but at the end he realized it was because a third party, the Terrans, were allied with everyone except him. The Terrans had defenses against the Dregin fleet so they would win if the alliance broke, but if the human player died the Terrans would win a diplomatic victory. So basically the Dregin were intentionally letting him live.
I dunno, I have a lot of say about both games but I really don't want to turn this into a which game is better competition. Especially not on the thread for one of those games. The original point I was making is that its funny that DW is pricing itself more expensive then a more or less AAA competitor.