In my opinion FFXIII in west will sell more at 360 because:

1)The Base is just bigger

2)The western Jrpg fans have moved for 360 because of the many 360 jrpg realesed

3)The old ps2 owners in majority now are 360 owners not ps3.

On PS3 of course:

1) And? Some multiplats sells better on PS3
2) ?
3) ???

Well....the thing is, FFXIII was announced for 360 a while later then PS3. So while its easily possible for the 360 to have more sales then PS3, its quite possible for the PS3 since many people thought it exclusive for a time.

HOWEVER.

Past trend have shown that blockbuster titles like this move many consoles for those buying the game on the day and week the game is out. So we don't quite know.

But because of the points you mentioned, I'm gonna say 360 for the West. 

on ps3,
1)the fact that the fan base is bigger doesn't mean anything
2) it doesn't change the fact that the only jrpg that sold well on 360 is lost odyssey.
3)there is no proof of that

I think we will have a clearer picture on this once Star Ocean 4 is released. As of right now both Xbox exclusive Square Enix games have bombed. If Star Ocean sells well which it should than I think Final Fantasy will sell well on the Xbox.

Who knows? We don't what the console base will look like in Spring 2010 or whenever the hell that game's gonna hit non-Japan stores.

It's not so much about the number of 360's in the US as much as the demographic distribution, and based on sales of games like halo and gow compared to sales of games like LR and IU,I say that the majority of the 360 crowd in the US isn't so much into JRPGs.

And don't pull the VC hasn't sold well on ps3 argument, that's a SRPG, wait for WKC.

In EU the PS3 version will sell more for sure
In US maybe x360 version will have the edge cos the install base for PS3 is way too small compare to x360

360 owners are not that interested in JRPG (like me)

Not that many people know about versus so that wont affect sales much but yes the ps3 will win in the west but the 360 ersion will put up a fight.

I'd say whichever gets more DLC. Fallout 3 for example has exclusive downloadable content available to the 360 and PC but not the PS3. For me, FF13 will be purchased for whichever console offering more DLC and usually, 360 gets the better end of the deal.