This is just my opinion...I'd pre-order Rebellion now, and just stick to that instead of getting Trinity...you save a little bit of money at the expense of having only a Beta version to play for now....

The strategy and gameplay between the two are so different that "practicing on Diplomacy" isn't as good of an idea as it may sound at first...

If you haven't played Sins for awhile, the Beta in its current state will probably be enough to keep your interest until the next update that releases Advent...

If you don't mind some beta difficulties I'd just go straight to Rebellion. It's got everything in Trinity and more stuff.

If you don't want to deal with beta bugs, then your best bet is to buy Entrenchment and Diplomacy, rather then Trinity (which as Zeta said is just the base game and Entrenchment/Diplomacy).

All expansions of sins including rebellion comes with the previous content. The difference with Rebellion is you do not get separate executables for Vanilla, Entrenchment and Diplomacy. Trinity however includes 3 different executables for the original game and each expansion. 

One way to look at purchasing Trinity right NOW is it will only cost you $10 IF you plan to buy Rebellion as you'll get $10 dollars off that purchase. The advantage here is you will get all 3 executables as well as Rebellion (assuming you pre-order now as the discount may not last forever).

Having all 4 executables opens you up to a wide range of gaming possibilities.

Diplomacy is currently the MP standard (might change with rebellion).

Mods that MAY make having all 3 executables worth your while:

Dawn of Victory (original version, not released but looks good, multi-player only mod)

Sins of the Prophets (Entrenchment)

Sins of a Galactic Empire (Entrenchment)

7ds beta (Entrenchment)

Maelstrom (All versions)

Distant Stars (Diplomacy)

Star Wars Requiem (Diplomacy)

Sins of the Fallen (Diplomacy)

Sins of the Fallen: Blood and Chrome (Battlestar Galactica) (Diplomacy, work in progress)

Stargate Races (Diplomacy)

Stargate Invasion (Diplomacy)

Star Trek: Sacrifice of Angels 2 (Diplomacy)

Babylon 5: Sins of the Younger Races (Diplomacy)

Goa's 4x mod (Diplomacy)

If you like mods the released mods alone are worth the extra 10 bucks.

I only listed active mods.

Dawn of Victory is not porting.

Sins of the Prophets is doubtful right now.

It's hard to say how long it will take the others to port, either way their current Diplomacy versions WILL work versus waiting to see if they MIGHT convert to Rebellion.