Honestly from Squareenix's perspective, putting it on both 360 and ps3 would only be good. PS3/360 would capture their respective markets in Japan. Xbox360 isn't doing good in Japan granted but it's doing better and there's mistwalker studio working as 2nd party for the 360 which will draw some jRPG players, a FF would only cap it off(essentially creating a cheaper solution), though I'm sure PS3 FF13 would get more support in Japan. As far as America Xbox360 is firmly rooted in the US and partially in Europe and other regions. To put it out for both 360 & PS3 would be a smart move unless they like to lose money and I assure you they don't, they don't even like to wait for money they want it ASAP. Putting it for both would open it to early PS3 adopters and to the large 360 fan-base without forcing them to buy the other system. To anybody who says "final fantasy is always exclusive" you need to also keep in mind, Final Fantasy 13's budget is a LOT more then any previous FF's as well so it's higher risk. Unless PS3's sales triple by the end of '07 if I were in charge of Square, I would 100% release it for both PS3 and the 360 if not JUST the 360.. yeah I'm crazy but think of the kick back you could talk microsoft into.
Essay Pro