extend the 7D with a 5D markII or markIII?
You will probably get a more technically informed answer from others than mine, but I would go with the 5DIII. I have the 7D, 5DII, and 5DIII. To me there is night and day difference between the 5DII and 5DIII.
While the 5DII is a great camera in its own right and in its day it was legendary, the 5DIII is much more than an updated 5DII in my opinion. I would pay the extra money for the AF alone, but in every way I get many more keepers and the exposure, color, and accurate focus still blows me away. It just has a "wow" factor that goes beyond the 5DII.
I want to keep the 7D for the reach/pixel density (birding), but in every other way the 5DII is my first choice. My 5DII never leaves the bag anymore.
That is just me though. I'm sure others will have other comments.