Upload & Sell: On
The right time to upgrade is a very personal thing. I've kept on the EOS bleeding edge for the last three or four years, because I've been pushing the capability of my gear; cameras in particular. Now I'm good with a 1DX and 1DIV, with a 5DII in reserve. A second 1DX and a 5DIII would be useful in some circumstances, but I'm OK.
Technology has come a long way. The step from Mk II to Mk III EOS bodies was a natural evoluvtion at the time, but it established a new generation for basic user interface procedures, battery capability, and computer interface. The Mk III bodies also have incrementally better per-pixel IQ, more pixels, slightly improved AF, an AF microadjust capability, and much better ISO performance at 1600 and 3200. The prices of 1DsIII and 1DIII bodies have fallen dramatically in the last year, especially the 1DsIII. ( )
The cost for stepping from Mk II to Mk III 1D-series cameras is now relatively low. Only you can decide if you want to take the step.