This is very simple. It will turn out like every other multiplatform release in the world. The console which the game was initially programmed for will get the best performance and quality. The consoles which get a port of the game will get slightly less performance and quality. Even a PS2 game that's ported to the Xbox can end up looking and performing better on the PS2, regardless of the fact that the Xbox is probably twice as powerful as the PS2. This is more often the rule than the exception.

As far as I (and everyone else) knows, FF13 is first being developed for the PS3.

Now, the 360 is not twice as powerful as the PS3, so work out the rest for yourselves.

Both consoles have an equal amount of RAM, by the way. The only difference is how the RAM is organized.