Considering your making the game on a middleware engine, then translating it to the PS3 and Xbox 360, the Xbox 360 version is not a port.
Look at any UE3 game: UE3 is a middleware engine similar to Crystal Tools, which is designed to be used for Xbox 360, PC and PS3 games.
So when you develop a game on UE3, you can publish it to 360, PC and PS3 somewhat easy, as your not re-building the game entirely for the given console of choice.
Crystal Tools and XIII is similar. Crystal Tools is build for the X360 and PS3. So if your building a game on that platform, how are you building it for one specific system and not both? I would agree that XIII's lead platform is the PS3, but that doesn't make the Xbox 360 version a port. A port would entail that the game was made for the Playstation 3 under a PS3-specific engine, and once the code was entirely completed on the PS3, it was then ported to the 360, and optimized for the X360. Rather, it's being done on Crystal Tools then ported to the Playstation 3 first, and the Xbox 360 second for the Western markets.
We've seen such scenarios many, many times this generation to where companies use middleware engines such as Torque, Unreal, and such to make a given game, and then send it to many different systems. This couldn't be done without a cross-platform development structure in place, such as what Square Enix is doing with Crystal Tools.
So I'm in the "neither version is a port" - the game is being made, clearly, for both systems, but the PS3 version will be finished first, provided the it launches in Japan first.
Back from the dead, I'm afraid.
