Here's the thing about the rumours the next xbox is going to be shown at e3 this year. From a business stand point it doesn't make a huge amount of sense. If they show it this year then really they want to have it out by Christmas. The 360 was shown in May and launched in November. So if it's shown at this E3 really it has to be released this year.
However think of all the MAJOR games. What about Halo 4? Launching a halo game against their own console would be insane, it would have to be shifted to Xbox next or dual released. Likewise GTA5 is this year. Assassins creed 3, Hitman, Tomb Raider, Devil may cry, Prey 2, Far Cry 3, Bioshock Infinite, Borderlands 2, Metal gear rising are all releasing later this year. Unless a HUGE amount of those games also have a xbox next version there is no way they would release a console to compete with that many big titles. It seems next year is the far more logical choice. Have one last big year now then next year have the games launching on xbox next in November time etc.
This time last year we knew a lot more games for 2012 than we know for 2013 this year. So holiday 2013 is my guess.