It's hard to say, it could go either way, really.
On the one hand, there's Konami. Yes, it costs extra money to make a 360 version, but they would almost certainly make that back from the sales. And I'm sure Microsoft would be willing to throw quite a bit of money at Konami.
On the other hand, there's Kojima. With the exception of a version of #2, every MGS so far has been PS exclusive. Kojima has made it no secret that he is a PS fan. PlayStation has always given his games success.
Kojima would never compromise the quality of his game for money. Never. He cares far more about making a great game than making pots of cash, like any good developer should.
So, if there was a 360 version, it would have to NOT reduce the quality of the PS3 version. And the PS3 is hard to develop for. It takes attention. Care. It's a lot easier to give it that attention when it is the only version in development. MGS5 would have to, of course, look better than MGS4, and run better than MGS4. The same engine could be used, just tweaked a bit. Whereas on 360, an engine and game would have to be built from scratch (unless they use the Unreal engine, and Kojima's just too awesome for that).
That's why I'm leaning towards PS3 exclusive.
There are a few things I know for certain:
-It will be on PS3. That's where the MGS fanbase is, that's where an MGS game sold 3.9 million and counting, before hitting Greatest Hits/Platinum/The Best.
-MGS4 will remain PS3 exclusive. Kojima hasn't given any of that crap about "We have no plans for a 360 version at this time" or "There is no 360 version in development", he straight out said "There will not be a 360 version".
-It will not be on Wii. The entire game would have to be redesigned, the graphics would become worse, and you'd end up with a crappy port like Dead Rising: Chop 'Til You Drop.
Now if it were to be on 360, I have a few requests:
1) The PS3 version should be better. Not the same. Not slightly worse. Better. Look better, run better. It should be the definitive version, because PS is where the series was born and belongs.
2) No 5GB install, please. Killzone 2 looked better than MGS4, and had no install at all. Sure, it wasn't as packed with content, but come on! Use triple layer Blu-ray, if you need to. No install, or at the most, 1GB install ONCE in the game, not six times.
3) And to the 360 owners, buy it. You'll be bragging for months about another lost exclusive, and how the PS3 has no exclusives, and no games, and is a money sink, and is doomed, etc., so the least you can do is buy the game you are using as a bragging right.
I would say 65% chance of being exclusive, 25% chance of being timed exclusive to PS3, 10% chance of being PS3 and 360 same day release.
(Former) Lead Moderator and (Eternal) VGC Detective 
