DCharlie takes the safe route by saying fanbois, but the truth is it is MS, because Sony's first party division crushes MS first party gaming division. I'm not only talking critical accliam, but the ability to move units in all three big regions. Too many people are overlooking Sony Worldwide Studios when talking about PS3.
actually, there are several ways you can argue this but they all rely on a couple of "IFs" which i'm trying to avoid because it's more hassle than it's worth, but just for you , here goes :
Line up wise, things obviously swing in Sonys favour if we take the idea that all 3rd parties go cross platform. That's just not arguable (if we ignore Nintendo of course!)
Some negatives for sony might be that the added power of their machine doesn't get used if some 3rd parties don't put the extra effort in to distinguish the X360 and PS3 versions of the game. I also expect the two machines to have various pros and cons meaning that certaing things can be done on one better than on the other, so (as mentioned by others) you get compromises on both platforms and the full extent of the machines aren't used. Therefore, the 'gamer' loses in the sense that they don't get the most out of their machine.
Now, so far - i'd say that this _would_ definitely swing things way in Sonys favour if this hypothetical situation happened.
However, there is one important factor that might change the whole complexion of this (and again, this incorporates several "IFs" so i know it's all not worth the post it's typed on - but , to complete the arguement). That factor is PRICE.
If all games do go cross platform and all games are pretty much the same, if MS drops the X360 retard pack to $199, and if Sony are going for an ubermachine in the $499 range, then that 'all 3rd party games on both platforms' suddenly makes the X360 very attractive (if you can live without Sony first party games).
Again, as said, this is all IFs and hypotheticals, but i'd restate the only sure fire losers will be fanboys (although we'd just see more intense first party battles)
One thing that is fantastic though, and it warms my heart to see, is that Oblivion is going to another platform and more people will get to enjoy an absolute classic. It's also nice to see that it has to be one of the least bashed games ever (given it was originally X360/PC) - the fanboys didn't try to jump on it.