Ultimate Edition is the base pack + the first two expansion sets. Argos Naval Yard is the newest one and adds a bunch of ship parts and technologies, but doesn't add anything huge (a new race).
If you're worried about the cost, stick with just the Ultimate. The fact is that SotS already has a lot of content and you won't miss too much with outArgos Naval Yard. (in fact, some of the stuff, like Overthrusters, I have yet to even get). If you decide you love the game, you can always pick up ANY at a later time.
However, picking up the full game + all three expansion sets seperately is a bit pricy. I recommend taking advantage of the Ultimate and the current sale. It's a 4X game, similar to Civilization, only it's in space. And each race is incredibly diverse and unique from the rest (can you say random tech tree?) The voice acting is horrid though
Though that's Paradox for you...