Owning a 360 and a PS3 is VERY redundant unless you are the type of gamer who doesn't want to miss anything when it comes to exclusives on any platform.
Of course lamenting the fact you spent 400 quid on a console that you currently have no use for = zero sympathy for zero foresight. It's not as though the release schedule for every console on the market isn't widely available on the net for anyone to check out.
If you KNOW every single release that's coming out, that makes it all too easy to tailor your choice or choices.
Not interested in MGS4 or GT4P or any of the current PS3 exclusives? Then why buy a PS3 now? By the time GT4 and FFXIII are released in about a year, the price of a PS3 will likely be down to 299.
There's always overlap when it comes to multi-platform gaming. The gaming PC I just built would have made my 360 obsolete if it weren't for the handful of 360 games still not available for the PC or PS3. But, since I like having the option of playing EVERYTHING that looks fun, it means having more hardware. And there's nothing new under the sun there, even if it means I have a Wii that gets turned on every few weeks or more, and a 360 that is the redheaded stepchild to the PS3.