I would say X3:TC. Essentially, they are all the same game. Egosoft has been working on it for many years now, adding more features and fixing more bugs. Every few years the put a bunch of new features together with new plots and release it as the next version. (This whole 'rebirth' thing is them actually dumping the original game completely and starting fresh.) So I would say X3:TC simply because it's the most advanced version of the game.
Stay away from the Terran start. Terran space is neat but they've got all those super-huge stations with super-huge docking lanes so it takes forever to park. It's much more enjoyable in there after you've bought a Paranid docking computer.
Thanks for the hints. Cool then.
I'll wait for another sale at GG and pick it up, then.