the 5d3's focusing system could play a part depending on what you are shooting. if its still life then no big deal. if its people then the Mrk2's unreliable focus points off center could become frustrating. you can reliable use focus points off center on the 5d3.
also, after this workshop what kinds of uses will you be needing for this new camera? chances are, if your interests are diverse, the Mrk3 will be more versatile and useful in almost any circumstance. if you have the money...get the camera that offers you the most.