I do most of commercial assignment in studio with controlled light and even in outdoors I have my lighting crew so this means I don't shoot high ISO images, at least for paid jobs!
To answer your question, I think 1DsIII even 3 years later still produce very nice files, and I have 5DIII in working today as well, but since I don't test cameras for job so I can't say which is better but looking thru the computer screen I would probably say they are very equal, but different.
Even with raw file and shoot under white balance present - "flash", as raw image view on C1Pro, the cr2 from 5DIII seemed a bit richer than the cr2 from 1DsIII appeared on C1Pro but then I can slightly adjust the C1Pro software so the 2 files looked the same.
With same lens, it is the same sharpness as I see them, similar contrast - so I would say at controlled light, they are equal.
For outdoor, very obviously the 5DIII has more pronounced richer color and auto white balance is probably the best I have tried, include D800E. And back to studio to develop the raw file, still the file without any preset looks to be richer on C1Pro, and 1DsIII file needs a little adjustment, but can be almost the same, so they are equal again.
But 5DIII has very very good focus except it is a bit too light for me. I would hope a camera may be a little bigger and a little heavier - or the size and weight of 1DsIII or 1DX still preferred.
The size and weight has probably no direct relationship to quality or may be they do, but it is to me more pleasure to shoot with a 1D series camera - for paid jobs, not becaue it looks better but feel a bit more secure, knowing they won't break. But 5DIII is a good choice for a few hours on street with no particular subject. Videos? I have other cam to do the job so it is not a point that would matter to compare them. But for people who cares, the choice is obvious.
So the verdict - in studio and heavy duty - 1DsIII. For people prefer lighter and smaller - 5DIII. For low light and street and video - 5DIII. To the end of world - 1DsIII.
BR,
Pngang