I believe in a MW2 scenario for FFXIII(Please, don't understand me wrong, I mean how sales will splitt between plataforms, not absolute number!).
MW2 will sell 70/30 between 360/PS3. I think 70% of FFXIII sales being in PS3 and 30% of sales being in 360 in a good guess for WW(already counting Japan).
So, it it reaches Darth expectation of 10 million it will be something like
7 million PS3/3 million 360. A pretty decent number for 360.
Anyway, I expect the 360 version to hold more legs, since the FF fanbase is getting the game for PS3 and the hype between the ps base is much higher than the hype between the 360 base.
Ask for your gamers friends, this game is much more popular between ps3 only owners than 360 only owners, and owners of the two consoles are likely to get the PS3 version.
In my case for example, I don't know any 360 owners that is hyped for XIII, even the ones that get hacked consoles aren't likely to get it, they are more likely to wait for GTA next, Halo:Reach or Gears of War 3. The PS3 owners that I know are super hyped for XIII,GT5 and God of War 3.
Another impression I have, it looks like the kind of cult player, the try to see poetry, art and good plot in games are more likely PS3 owners. At least my 360 friends are more likely "Oh, I killed so many people yesterday playing MW2!" and the ps3 friends are like "That final battle between Snake and Liquid were so emotioning!"