I think X3:TC is more like 80% sandbox, 20% story missions. It's getting hard to say those numbers now, though, because of one particular story arc - The HUB. In this arc, you gather huge quantities of items between bits of story. So you get a little story, then you have to donate half a million units of ore (which will probably take you weeks). Then you get a bit more story and get another massive request. The effect is to make something of a "background" story. When you get to a new step, you gear your empire toward the new product, then just let it produce while you go off and do other missions/building/fighting. A week later, you finish that step, get some more of the story, and change gears again. I've never seen anything like that in a game before.
X3:Terran Conflict is now in the "real" stores in the USA, not just on Steam. The "Gold" edition (which is X3: Reunion and X3:TC bundled together with a music CD tossed in) should be out in the EU now.
Personally, I would stick with X3:TC. Each new X game is built as an improvement to the previous one. You might as well get the latest & greatest. So what if it costs you $20 more? If the game takes 200 hours, it's only an extra 10 cents an hour.