It was earlier hinted on the 1st Production Department Premiere Trailer that Jecht will be part of Cosmos' side and Terra, Cloud and Tidus will align themselves with Chaos! Just recently, asides that Square-Enix Europe releases a subbed version of the said trailer (verifying what most fans wonder), the official covers of the official Dissidia 012 [duodecim]: Final Fantasy Soundtrack was released on their official website!
All characters found on Cosmos' side is featured on the front cover (left) and oddly enough, Kain Highwind of FFIV who has a long history of backstabbing and being mind controlled by Golbez is found on Cosmos side!
While all characters found on Chaos' side is featured on the back cover (right). Not surprised with Terra and Cloud being part of Chaos side, you know, they have the history of having no memories of their own and oh, Terra has this history of wearing a certain Slave Crown. As for Tidus being part of Chaos' side, based on the subbed trailer, it seems that he is brainwashed to join their side.
The official soundtrack will be released in Japan this March 2, a day before the actual release of the said game. If you are wondering what are the songs found in the soundtrack, based on the tracklist found in their official website, most of them are remixes of old Final Fantasy tunes with some original tracks made for the game.
Special thanks to Siliconera for the info!