I liked both Tachyon and X. Both are excellent story-wise, but Tachyon is better if you're just looking to find something to kill a few days to a week or so on. (X requires weeks to play).
Don't get me wrong, X can be a good game, but if you're just starting (especially if you're in the first game of the series) you may want to have a book handy to keep the boredom at bay.
Tachyon - had a nice variety of ships and a fairly good story, even if the play was fairly linear and had limited side-missions and whatnot.
PROS: beautiful ship designs and colors. Wonderful artwork.
CONS: almost no side-quests. You play like 2 missions and then have to choose a side. At best you play the game twice before you've essentually seen the entire plot. Replay value then drops sharply (unless you didn't manage to buy/fly all the ships).
X - is more open in terms of story and missions, but the money-saving and slow-flight aspects (especially in the first game) made it extremely frustrating. Graphics are "more realistic" in terms of ship-design and base design, but the designs are boring, dull in color, and the physics of long-term flight over great distances is basically Slower-Than-Light (if not turtle-speed).
PROS: The enemy (Xenon) were very hard (like playing WC on its hardest or
2nd hardest setting, at least until you max yoru ship out). This make the game a good challenge. There are lots of side-quests/missions, a fair number of buyable ship-upgrades, and different ways to make your money (from legal trade to buying and making the goods yourself to piracy).
PROS pt.2: Multiple games (X, X2, X3, etc). Descent story (with an interesting enemy who isn't the "typical war-loving aliens" that most games feature).
CONS: Money takes forever to start flowing. Battles are equally hard until you get a few upgrades.
CONS pt.2: The game uses realistic speed/distance for travel, and sub-light travel is redicuously slow (even after you get the "time compression" thing, it still takes ten or twenty minutes).
CONS pt.3: Frequent saves are required ("frequent" is a joke... the STL travel renders your save chances easily over an hour apart). Die (or worse, forget to save) and you can that your game has suddenly set you back SEVERAL HOURS OF HARD WORK all for nothing.
CONS pt.4: you can easily find that a "small trade run" can easily take you ONE HOUR PLUS. One way to describe it is to imagine driving from Earth to Saturn in a car (that is to say, your super-fantastic ship barely crawls along at 300 km/h. And considering that the journey is like 50,000,000,000 million km... well, you better hope your ship has a bathroom on board cause the next gas-station-in-space be a week away at that speed.)