I'm pretty sure (not counting elite), X series are the longest ongoing space sim games. You can skip the first games because they're very old, and they have very confusing naming.
X3: Terran Conflict is still considered one of the best space sim games ever made (even though it was a pile of mess on release, but that seems to be a "thing" for Egosoft). X3: Albian Prelude is the expansion, but it's the same universe, same graphics, same engine, just a total different setting.
There are TONS of mods for the game, one of which is X3 - X-tended that adds a few gigabytes of addons, insane graphics improvements and tons of new systems. Everything free and fan made.
The newest one (X: rebirth) had a big flop on release and it was totally broken and unplayable. Luckaly, Egosoft did a great job of patching the game and releasing an extra DLC. The game at this point is bug free (except a few silly NPC movement now and then) and very enjoyable. However, all X games have a steep learning curve - but they really feel cousy and look amazing.
If you're interested, take a peak of my X3 screenshots
here and
here, and some from
X:RebirthJust like Elite - they are really slow to get into, and the story pretty much never holds your hand nor tells you exactly what to do. You need to figure stuff on your own.
back on topic - now when I looked at it, you're right. Both games are pretty much made by the "same" people. No wonder they look alike
