My understanding based on what the developers have said is that Rebellion will be a "stand-alone" expanstion that will include all of the previous content from the original Sins game and the two expansions. In other words, you could just purchase rebellion and play that without ever having owned any other Sins games. I think there will be a $10 discount on the Rebellion download for people who already own the Diplomacy expansion (aka "Trinity").
How badly do you want to play this game and how much time will you have to devote to it? It's possible that it would make sense to just purchase Trinity now if you'll have large amounts of time to play it over the summer.
