Why? Besides those of you who only have a 360, this makes no sense to me. I see no clear advantage in doing so; I could see this on some games where the ps3 version was technically inferior as is the case sometimes, but in all likelyhood, since XIII has been in development for 5+ years on the ps3 and will supposedly be ported in only a year to the 360 I don't think this will be the case. Now, I'm not saying that the 360 version will be inferior; that remains to be seen. What I am saying is that in the best case scenario, there is no advantage to owning the 360 version over the ps3. In fact, even though the advantages would be small, the advantage still lies with the ps3:

1 blu-ray disc
ps3 has better audio
versus still exclusive (though likely not unfortunately)
ps3 lead development (that is going to be an awfully big game to port in such a small amount of time and get everything sound, especially if you're dividing your staff among the localization as well)

I doubt many people with both systems would buy it for the 360.

1. bluRay disc-- doesn't matter for the game itself?  The 360 verion might come on multiple discs, but meh.  Not really a big deal.
   
2. Better Audio-- only better if you have 7.1 and the game supports 7.1
3. What does FFvXIII have to do with it?
4. Doesn't mean it will be better

So yeah, I guess my answer is why not?  None of your "points" are really valid except if you have 7.1 (which I don't) and it supports 7.1.

If I end up getting the game, I'll just get it for whichever one is better.  You can assume all you want that the PS3 version is better, but there's no way to know until the game is actually released.

-edit-

And assuming both will be the same or about the same quality (because I truly believe they will be), I would probably get the 360 version because I prefer the 360 controller and achievements.  And I know, the PS3 has trophies, but I just don't like collecting trophies nearly as much as collecting achievements.

 1 disc is irrelevant, Versus being exclusive to the PS3 has absolutely no bearing on FFXIII at all, and PS3 being the lead development is irrelevant unless there is confirmed issues. The Crystal Tools engine has been up and running for the 360 for quite some time now. The sound is the only real difference you bring up, and most people won't have a set-up to utilize it. Honestly why not pick up the 360 version?

Geez.. Almost one year later and people are still butt-hurt about FFXIII going multiplatform... well.. here's to another year!! :)

@twesterm; I'm assuming for both titles to be equal in all aspects possible (primarily graphics); I'm merely stating that in regard to other aspects the ps3 should have an advantage; no matter how small, an advantage is still an advantage, and it doesn't make any sense why anybody would disregard that. Now I can agree with you on waiting to see which version will be better. And as far as my comment on Versus I was pointing that out because some might wish to have both titles on one system for a sense of parallelism (though likely not many people are as uptight on me on those aspects, but still...)

Surprise!!!

I don't have 7.1 surround, and I'm one of those wierd people who, for whatever unconcious reason, seem to believe in a better deal when getting a game on multiple discs. And achievements are more compelling here then trophies.

However.

If the 360 version is by any means higher priced then the PS3 version, I'm getting the latter. Otherwise, I'll just be adding FFXIII to my growing list of great JRPGs on my gamertag. Obviously, if I get Versus, it really won't matter which system I get FFXIII on. In fact...1 for my PS3, one for my 360 :P