PS3 alone. The PS3 has passed the 5 million mark in Japan which is more than enough to justify the development of PS3 exclusives. Games like Fist of the North Star and Yakuza 4 show that any big PS3 title makes the PS3 the best selling console in Japan (including handhelds) for a period of time.
Persona games usually only sell what games like Yakuza 4 and Fist of the North Star sold week one on PS3.
Also, the effect of making the game for 360 would be negligible at best and putting the game on a second console (without a user base to buy it in substantial numbers) might drive up the costs of the game's development a little bit.