Well, it's not like I'm an authority on 4X's or anything like that, but since you asked: Compared to GalCiv2 I found SotS... probably roughly as fun, just in a very different way. In GalCiv there's way more detail on your planets and what you build, etc. In SotS there's way more detail on combat and the meaningfulness of your ship design decisions (design matters in GalCiv2 but there's no tactical combat).
I get the impression that SotS is the better game overall, I just haven't gotten far enough into it to realize it because I've never fully gotten a handle on the combat system such that I feel like I'm not making stupid novice mistakes. Whereas I pretty thoroughly figured out GalCiv2 from top to bottom (though they came out with another expansion after that and I don't know much about terror stars, etc).
(none of the above applies to SotS2, as I haven't had any experience with it other than to realize that they're still working on getting it to release quality, though perhaps they don't see it that way)