been following that game as well for some time. it has more tactical aspects than GC2 (detailed attention to ship battle mechanics), whereas its strategic elements are somewhat abstracted (no individual planetary buildings, just overall sliders for 'infrastructure' and 'terraforming' etc).
it also uses a different approach when it comes to alien identities... whereas GC2 follows the traditional formula of having lots of highly customizable races that are mainly differentiated by plus and minus attributes, SotS focuses on only four races, and extensively differentiate each one of them qualitatively as well, such as mode of travel (very interesting variety, mind you) and special/restricted technologies. Also, each of the four races have amazingly indepth and believable racial profile & history.
i don't find much info abt the level of complexity of the AIs. the
wiki site mentions abt AI awareness & adverse reaction to betrayals, even those that occur to other civs. And of course, more complex AI is expected to be involved in tactical battles.
plagiarism? IMO as pointed above, these two games follow very different approach despite sharing the same genre - although SotS also can be considered as partially RTS/RTT.