I haven't heard that the 6D sensor will perform better than the 5D3 sensor in low light. I would be shocked if it does in fact produce less noise at high ISOs. So if the 6D does perform better in low light... then I would be inclined to sell my 5D3 and downgrade to a 6D... but then I can't get past the fact that the 6D doesn't have a joystick or rear wheel for shooting efficiency so getting a 6D would never happen for me... There's no time to waste fussing around with the menu system to change critical settings like aperture and AF point selection at weddings. I still intend to rent a 6D once it's released just to see what it's all about and compare it to the 5D2 and 5D3... just to satisfy my curiosity
I have heard a similar rumor about the T4i having better high ISO performance than the 5D2 and that turned out to be false.