A completely different game which will rule just as hard as FFXIII.

I can't see VXIII remaining exclusive. I can link you to twenty articles on how Square kept saying they would not jump ship with FFXIII. I don't particularly care since I'll buy both anyway.

I haven't seen any threads on Agito PSP...why?

I doubt it will backfire. I expect a skewed version of what we saw with DMC4 and GTA IV (assuming the whole thing stays PS3 only in Japan):

PS3 will sell more WW thanks to Japan and Versus (360 would need to sell a lot more to balance out loss of Japanese sales IMHO)

360 will sell well and may sell more than PS3 in US.

360 will likely gain a modest console spike (I think loyal fans will go with a PS3 if they are making a new console choice but the other 360 jRPGs might swing things a little and drive a modest spike)

PS3 will get a bigger console spike WW, particularly thanks to Japan, where I expect a much bigger spike than we saw for MGS4. This could be helped if Sony get White Knight and other jRPG's out or well marketed before and affter FF release to push notion of playing multiple big jRGPs on the platform.

No one really loses, everyone gains something, and the biggest winner is probably SE as I doubt the current positioning will cause any cannibalization between PS3 / 360 versions but will deliver more sales than an exclusive on either platform.

every single FF-game sold on the 360 is a "not-sold-on-the-PS3-game". It is that simple.

For your implied statement to be true everyone who buys it on 360 would have to have been prepared to buy a PS3 if they didn't own one instead.  I really doubt that.  Most 360 purchases will be existing owners more than happy to get the game since its available (and assuming its good) but I believe that had it remained PS3 only many would simply have passed the game by.

PS3 owners will not get a 360 over this (not enough to matter statistically) while 360 owners are unlikey to get a PS3 instead (obviously... unless they are desperate for Versus).

Your statement assumes 100% cannibalization - that's very unlikely to be the case.

I don't think that is true at all. 

Reason 1:

Look at how many Nintendo owners never played FF after VI and how many new gamers started at VII or X.

Not every FF sale is to an existing FF fan.

Reason 2:

Not every previous FF buyer would have necessarily bought a PS3 for FF.  Maybe they had to choose between FF and Halo, or all those other JRPGs.

Heh. I don't know that Final Fantasy XIII going to Xbox 360 will backfire. I barely heard anything about FF XIII Versus before FFXIII went multiplat. Now all of a sudden it's FFXIII VS that's the big deal.

All FFs spin offs are just for the fanatics. I own all Finals in the series, never have I bought a spin off so really, FF13 does nothing to MS

Since Microsoft gave me so many rpgs, I will get Final Fantasy 13 on the xbox360. (Even if I have a ps3)

So that is one sale minus for PS3, and one sale plus for 360.

Who would have thought someone like me hating microsoft last generation, would love microsoft this generation.

Also I misread the question: Final Fantasy Verses is a Spinoff....I will get it but I have no faith in it.

whatever the real story is, the fact remains that:
sony has devs working with S-E. S-E is about the closest thing to a 2nd party there is without actually being one. one can only wonder what kind of things they have worked out together.

The whole situation is fairly strange.
Sony owns stock in SE so that means every copy sold on MS's console means money in Sony's pocket.
There are also small incentives to stick with the PS3 as the platform for the game such as...
1. PS3 version gets a demo with Advent Children BD disc that is said to be exclusive to them ONLY.
2. FFXIII is part of an on going mini franchise SE is starting, as they said, they have begun to elaberate on each individual FF title instead of just moving on. The FFXIII series is going to last this entire Console Gen. FF versus XIII is not a "Spinoff" per say but more of an entire new game that takes the title in a different direction. There is little connecting the 3 (announced) games to each other other than mythology.
3. There is still another FF to be unveiled as the name FF Hearasis XIII has been copyrighted by SE.
4. Fans of the Kingdom Hearts series are bound to stick with the PS3 versions cuz they are made by the same team, and will use similar techniques.