Better lenses are more important than a better body... BUT, I really did notice more of an improvement in image quality jumping from crop bodies to the 5D2 than I had in any of my upgrades before or since. Lenses just work better when you don't magnify the image as much. (Of course this was for everything but bird photography - which got harder until I picked up a crop body just for that).
On to the original topic. I'd get whatever full frame body you can get cheapest then spend the rest on glass. If the 6D and 5D2 are close on price then I'd always go for the newer model, but I suspect you can find some pretty good deals on an older 5D2 these days. Of course the 5D3 is a better camera, but I am not at all convinced it's a $1500 better camera.