GTA sold well on the xbox because of the type of game it is, and the fact that the average 360 user is in their mid-teens. It being previously a PS game had nothing to do with sales. Everyone already knew what was in store for them when GTA IV was released and were getting it regardless of what system they had.
FF13 is different. Since Japan is dominated by PS3s and the FF series, they have already given it a near 2 million boost in sales.
Also, FF13 for 360 will not be available in all areas of the world. It many, it will be a ps3 exclusive.
And alas, lots of people own a PS3 specifically because of games like Final Fantasy and Gran Turismo. IF and only IF 360 sales of FF13 even approach the PS3 sales, it will be in the U.S. only, where there are several million more 360s in circulation.