The... (counting) third expansion falls more on the quality vs. quantity side of things. You only get four new systems, but they are unlike anything else in the game; 10 new ships with a radically different aesthetic (including the Manticore tug, which finally lets you "come to grips" with space-garbage; yay!); a new plot line involving gambling, pirates, scavengers and spooky space phenomena; new player starts that I can't comment on (I'm still playing the same game I started when Foundations came out); and -- the free, mandatory 5.0 patch.
Which is where the need for "more characters" comes in (because this review is capped at 2000 and just the list of improvements is 5385 characters long; that's on top of bug fixes).
Between the two, you get the sleeker "E" series of Paranid capital ships, economy and game-play refinements, new mission types, bug fixes, and more surprisingly wry dialogue from my favorite Boron scientist of all time.
For existing players -- the only downside I've spotted so far is the load times are slightly longer. And if you've been on the fence about getting into X4 -- the game's now in the best shape it's ever been in.