The 5D 2/3 has a bigger sensor and larger pixels so is able to render more detail and wider dynamic range... I don't know how much difference you'd see on the 800px sized images I post online but on a nice big print you'd appreciate it.
The dynamic range difference is minimal to none. The 5D2/3 may render colors a bit better, but that's primarily at the higher ISOs (the 5D3 is, it seems, particularly good at this).
Detail is another matter altogether. That depends very much on the lens you use. Use a sufficiently sharp lens on your 7D and the end result will differ from the 5D2/3's only in terms of absolute resolution (those cameras have more pixels than the 7D does, 22 megapixels versus 18), and that will make a difference only at the very largest print sizes. For landscapes where you need a wide angle, the Sigma 8-16 is a killer lens.
Here's my contribution, such as it is, to this thread:
IMG_6397-1.jpg by n2185x, on Flickr