Sword of the Stars is good. After the two expansions it is better than Sins. If you didn't know, it is a turn-based 4X with real-time combat.
There are originally four races, and each expansion adds a new one, plus new techs and gameplay features. The latest one even adds fighters
. Unlike Sins, each faction feels and plays vastly different from each other even though like in any 4X all races have a very similar tech tree. Not to mention there are some totally kickass mods.
There is a storyline, but really only a storyline in the sense Sins has a storyline. You will not care at first, but once you get involved and read the stuff on its forums, especially the various racial stuff, you will be hooked since there is so much more background and fluff than in Sins. SotS is much, much more immersive than Sins.