Can you try out the MKIII for yourself - why not see if someone local to you has the MKIII and see if you can trial it and do your own real world test, and actually see if you can notice a difference ?
I believe the MKIII version is an improvement over the MKII if you are using the newest lenses as Chris says, but don't know by how much / if it is worth the outlay - obviously, you could sell your MKII which would reduce the outlay and potentially make it a minimal upgrade cost...
Be interred to see the results, if you do manage to try the new MKIII over the MKII