The complete version of Sword of the Stars has enough to keep any 4x junkie happy for a long time.
SoTS: The Complete Collection is superior in leaps and bounds to that original little 4x game that could. There are now six races compared to the original four, each with their own look, technologies, strengths and weaknesses. No two races play exactly the same, and each one offers something different to the player, particularly with the inclusion of race-specific techs.
The randomized tech tree (while not very appealing to a completionist like me) ensures that no two games will be alike, and it has been greatly expanded over the course of the three expansions. There are a plethora of new hull types and weapons, allowing you to make use of things like jammers, decoys, and electronic warfare - even propaganda to weaken your enemies' morale. Diplomacy has been greatly expanded, allowing you to form specific agreements with other factions while requiring you to research multiple levels of each race's language before you can head to the negotiating table. Research enough and you can even attract alien immigrants to your own planets. If you prefer warfare, there are brand new weapons, ranging from combat drones to special gravity-based disruptors.
There are different infrastructure and map options as well. The number of map types and scenarios has been increased, and once in-game, new techs and construction ships allow you to design and build space stations around your planets - even the dreaded and deadly asteroid monitors. The Born of Blood expansion introduces the trade economic system, allowing you to build freighters and escort ships in sectors that you've secured, as well as the ability to raid enemy shipping. If you're looking for a challenge beyond simply competing with the other races, there are now dozens of random hazards to be dealt with; many of the new encounters are not only powerful, but move at will across the map, causing destruction in their wake. Some will even spawn duplicates from their current locations before moving, seeking to spread their influence to multiple systems.
Given its reasonable price and the current, extremely buggy state of SoTS II, the Complete Collection version of the original game is definitely worth picking up - IF you're one of the folks who's not played it yet and would like to get the whole deal up front. Giving you access to the core game and all of the expansion content, the Complete Collection is a definite bargain.