Depends on what kind of photography you enjoy or do.
For action, aviation, wildlife, photojournalism, sports; 1D-IV is a good bet
For everything else; - 5D-MKIII
If you do a little of everything; get both. :-)
And I only have a 1D-IV but want very badly a 5D-III. I would have said the 1D-IV also for low light performance but apparently the 5d-III has it beat there also.