As the above poster mentioned, there is a fair bit of difference.
Sins of a Solar Empire has no campaign, for example. Meanwhile, Imperium Galactica 2 is a shining example of how a 4x game should do a singleplayer campaign (ie like Alpha Centauri).
Sins of a Solar Empire is more of an RTS with 4x elements, but is completely worth your buying dollars. It looks great and is quite addictive despite the lack of a reasonable campaign. I'm no pro at the game, but your comment about micro is apt for Sins. I'm sure mp addicts will tell you there is tons, but the most I've done in that game is position a Capital ship to more effectively deliver broadsides. Oh yes, there's also a retreat button, so you can more easily pull units away from combat with less clicks (shift+r).
So yes, it compares favorably to IG2, but it has *no* campaign.
Regarding Nexus... it's not at all like IG2. It's fun, but it's more tactical than strategic. Hard to say, may be what you're looking for.