I believe it will sell more on the 360 in the west, but Japan will put the PS3 version ahead worldwide.
I strongly disagree with the brand recognition argument, and after the many examples of it not really panning out I'm surprised it's still accepted almost as if it's a fact. It didn't work out for a number of formerly PS-exclusive titles, it certainly hasn't worked out for the PS3, and especially if Japan is excluded, it's not going to work out for FFXIII.
Final Fantasy has weathered another brand recognition storm before, when they switched from Nintendo to Sony, and they did fine. I think the problem here is that people are forgetting games sell systems, not vice versa. Final Fantasy didn't collapse even though it was regarded as essentially a Nintendo franchise at the time. Gamers will follow Final Fantasy wherever it goes, wherever individual gamers decides are the best choice for them. This could mean that they will just stick to whatever system they have, or whichever system is the cheapest. Most gamers will make the choice based what is best fit for them, while a very small portion will say FF = PS. My belief is that this advantage will be toward the 360 by late 2010 (when a non-Japanese FFXIII release will likely happen).
Short of a massive surge by the PS3, the userbase difference in the west is likely not going to go below 8 million, and could potentially be 15 million in the 360's favor (it is currently just shy of 10 million). With that kind of installed base advantage in the west, combined with a lower price, alond with gamers' demonstrated tendencies to ignore brand associations this gen, and FF's casual appeal that transcends genre fans, I see the 360 versions outselling the PS3 version in the west.