That flight school is only if you need to learn how to fly and aim and collect cargo and such.

The games are very much sandbox with "smaller" plots and not one big one. You can do or leave them as you see fit. AP brings some changes: improvements, changes that come with both pros and cons, the war zone (NEVER have anything but your personal ship in there, including ships, satellites and factories) and the stock exchange (just another way to make a lot of money fast). They are stand-alone from each other.

Balancing is different in X2, X3R, X3TC and X3AP so what is good or works well in one does not in another.

I think the plots of previous games matter only a little and each game can be played without knowing anything about the others.

All game starts decide what equipment you start with and your relations to factions. The 'custom start' has all (almost) plots disabled.

I'd agree with Themken. Each game's plot is fairly inconsequential in enjoying any of the others.

As for which to start with, both X3TC and X3AP are full sandbox and pretty light on plot, but if you want any sort of "campaign", Albion Prelude (AP) has a bit more structure to it. AP has two main plot starting points, depending on if you go Argon or Terran, whereas TC doesn't really have a "main" plot, just a handful of long plot lines to be discovered as you explore. Of course, the plot lines in both games rely somewhat heavily on who you ally with along the way.

Terran Conflict (TC) can be played a little more casually, at least from game start. As Themken alluded to, the war zones in AP can be a little tense early-game until you've built up your fleet—though they aren't a bad place to play cleanup to bolster your munitions or make some extra cash selling off spare missiles.

If you prefer to ramp up your battle rank with combat missions, I'd say go for AP. If you prefer to build a trade empire, TC might be the better choice.

The biggest TC plots (Hub Plot in particular) tend to require a focus on building and trade. Beyond that, there are plenty of shorter mission-sequence plot lines you can get for the various corporations that usually entail caravan escort, station defense, etc., and a few semi-large race/faction plots that can land you huge favor or a sweet ride. But, in large part, it's about creating the story you want through choosing your allies and boosting or suppressing the supply chain.

As AP centers around a war, it should come as no surprise that it starts off with a bit more action going on. Of course, you could play AP as a builder too, the major plot lines are just a little more geared toward combat.

Then there's playing as a pirate. If that's your bag, most plot lines go out the window anyway, and either game plays about the same—though I'd probably take AP for the extra sectors and over 20 additional ship types.

Of course, there are mods out there to bring the TC plots into AP if you want to mix them, but the crossover tends to create too many plot bugs for my taste.

Notes:
• AP has the Stock Exchange, which can net you quick cash early-game. Always felt like a cheap hack to me though, so I just pretend it's not there.
• Ship hulls are (much) thicker in AP—take note of that when picking a weapon loadout.
• Game start ("class" selection) determines race/faction relations, but those change easily enough after a few pick-up quests.
• Humble Merchant allows for a peaceful start with (mostly) good relations across the board in either game, but is a slower start than the rest.
• Both games have pirates and Xenon as common enemies, but to my recollection the Kha'ak are only present in TC (in case you happen to like their ships).

Thanks to both for the answers. So I will begin with TC for now, as I want to do some merchant stuff more than combat. Have a nice day!

Sorry for the come back, but I forgot to make another question.
What is the most fun X game, the one most alive and to play more hours with, in your opinion?

That's a tough one, as you can easily sink thousands of hours into TC or AP—between the various starting scenarios, play styles, and mods available there's a lot of variety to be had.

After a few early restarts, I opted to build my trade fleet in TC, so I've probably put about twice as much time into that—mostly floating around in a stocked Chokaro/Zephyrus with Falcon Haulers, sending trades, building stations, and stockpiling (the Hub takes a LONG time).

I prefer AP for action though, so I've spent a good bit of time there, mostly in a Split Dragon with a trail of TMs for sector policing, or a Springblossom when I wanna get into dogfights. Yeah, there are bigger and badder ships, but I prefer agility. Honestly, I usually spend the first 10-20 hours at the start of most games zipping around in an M5.

I've played most of the major plot lines at one point or another, but I usually just blow them off and role play my own sandbox story—even got one game where I'm just a lowly taxi driver who secretly sells a-hole passengers off into slavery with the pirates. It's a shame that those troop transports and luxury liner riders who "have to get to a meeting" don't actually affect the game economy in any way, but point is that each game is what you make it. So once you really get a handle on things just make up scenarios and then play them out =)

--Mods--

Always install the official Bonus Pack, and I'd recommend the MARS Fire Control mod for any play style in either game—turret and drone management just kinda sucks without it. I prefer a cockpit mod as well, particularly for a more aggressive play style. It blocks a bit of the view you would otherwise have, but its makes it feel a lot more like a flight sim which just seems right when the action heats up.

Other than a few experiments here and there, I run TC near vanilla with Bonus Pack and MARS only. For AP I run those two plus XRM, as I think the rebalanced hulls, shields, and weapons make for better battles.

--Side Note--

I only got 30 or 40 hours into Rebirth before I couldn't take it anymore—so not that one ;)