soase is much more of a RTS than a 4x... so, while it has a few similarities to GC2 and Civ4... its more like... CnC or Homeworld. I personally have gotten rediculous quantities of play hours out of Sins of a Solar Empire, definately in my personal top 3 of hours played on a game, if not first.
I highly suggust getting sins (just make sure to get the whole package... trinity... it feels so much more complete than vanilla, and even more worth it.)