The decision was made fairly recently, although I imagine the option had been on the table from the beginning due to the less than stellar adoption rate of the PS3. That's keeping in mind that development on FFXIII began before the PS3 was available at $399 and the subsequent increase in sales that the lower price point stimulated.
I hope nobody is realistically going to think after playing the final build of FFXIII on the PS3 platform that "this game could have been SO much better if it had been exclusively coded for the PS3."
My guess will be about zero people will be thinking this as they are enjoying the game.
With every multi-platform game being coded by any developer worth the skin they're printed on, they will take full advantage of every capability a given hardware platform provides within the limits of their abilities and the resources at hand.
(Off topic, but FFVII/N64 analogies are completely unfounded here since the decision to develop the game for the PS platform due to the heavy FMV resources meant that FFVII never showed up on the N64 AT ALL.)







