A Bridge Between the Old and New X Universe
X³: Albion Prelude plays in a time between the old and the new X Universe. It shows a universe in flux, and prepares players for things to come. The old X Universe is passing; a new X Universe is about to be created.
War!
The X universe is undergoing a period of massive change. What started as a conflict of interest between the Earth and the races of the X Universe has now escalated into a full scale war. Play a part in a war scenario bigger than anything the X Universe has ever seen before!
Corporations Compete for Control of the Future
With the war as a backdrop, corporations are vying for power and are driving the X Universe in a new direction. A technological breakthrough will soon allow massive accelerators, forming highways in space and allowing the economy to expand on an unprecedented scale. Highways, a key feature of X Rebirth, are not yet ready, but you can see them being built and participate in the power struggles around the construction of much larger self sustaining economies.
Feature overview

A new plot throws you into the power struggle between corporations at a time of war.

More than 30 new ships and stations

New Stock Exchange feature allows advanced investment strategies

Improved graphics quality setting for high end machines

Improved UI introducing charts to visualize economic developments

Support for head tracking

Note: X³: Albion Prelude will not be compatible with Save Games from X³: Terran Conflict.
Available on Steam
X³: Albion Prelude will be available December 15th on Steam for direct download in English, German, and French languages (voice) and Italian (text only).
If you buy the X: Superbox, or already own it, X³: Albion Prelude will be available as a free download via Steam.
Available for Mac
A standalone version of X³: Albion Prelude will be available from the MAC Appstore and Deliver2Mac soon.

I know this series is being talked about right now in the Steam Daily Deals thread, but I don't want to clog it with questions about a single game.
Some time ago I bought the bundle that includes all the games in the series, from X-Tension to X3: Albion prelude. Never having played one before, I read the latter games include much better gameplay than the former, and that the storyline wasn't powerful enough to warrant playing them in order, so I decided to start directly with X3. Foolishly, I installed X3: Reunion, the first of the X3 series; then other shinies caught my attention until now, when I feel like playing some space trading/fighting game.
(Why foolishly? I have a limit on how many gigs I can download/upload in a month -- 70Gb between me and my wife, to be exact. Modern games take between 6 and 12 gigs to download, give or take, and that's a sizable chunk of my monthly allowance; so I have to think long and hard before downloading any of them.)
Anyway, my question is: Assuming I have enough bandwidth to download one more game before my monthly allowance is renewed, should I:
1) Start playing X3: Reunion, which I already have installed, and use that bandwidth to download other games acquired through the recent sales. Variety! Yay!
2) Forget about Reunion and download X3: Terran Conflict instead. More gameplay options! Yay!
3) Forget about Reunion and Terran Conflict and directly download the X3: Albion Prelude instead. Even more options! Huzzah!
The thing is, I don't know enough about the games, or what was improved and added between them, to make an informed decision; hence my question. Also, is the Albion prelude stand-alone, or do I need to install Terran Conflict first? Because I somehow thought it was a small add-on, but Steam apparently lets me download them separately -- and Albion is bigger than Terran Conflict by more than a Gb. This is so confusing.
Please help me! You're my only hope!