I have both bodies, the MkIV ive owned for 2 1/2 years and the
5D MkIII for about a year now.
When I first bought my 5D i was so impressed with it that I put my MkIV up for sale. I thought that as an
allround camera the 5D was without equal (1DX excepted). I still consider the 5 the best value for money camera that Canon has made for a long time.
5D MkIII
PROS.
AF is better than the MkIV overall.
High ISO performance is better than MkIV.
More options for customising various buttons to get the camera how you want it.
Much easier access to Live View.
AF to f8
Better screen to review images.
Build quality OK.
CONS
Only 6FPS. This is the biggest issue for me.
Inferior build quality.
No crop factor.
1D MkIV
PROS.
Slightly more reach with the crop factor.
AF still very capable.
Superb build quality.
Faster frame rate has its uses.
AF to f8.
CONS.
Review screen too small.
ISO performance not as good as 5D
Not quite as user friendly in its customisation.
I didnt sell my MkIV and I'm now glad of that fact. Its still a very capable camera and produces superb images. I tend to use it when i want a bit more reach or the faster frame rate.
Dropping from the 1.3 crop to FF of the 5D isnt such a big jump. Cropping the 5d image to the same viewpoint still leaves you with plenty. 13 MP against the 16MP of the MkIV.
The MkIV is faster to initially acquire focus but it doesnt hang onto it as well as the 5D.
The MkIV ISO performance isnt quite as good as the 5D. The difference starts to become quite noticable above ISO 2000 and I wont use the MkIV above 2500. On the 5D i have A3 prints taken at ISO 2000 and they are superb. I also have A4 prints from ISO 5000 files that are very good also. What I do like about the 5D files is that they seem very easy to process. Even high ISO files in LR4 come good very easily.
The AF on the 5D isnt quite as good as the 1DX but theres not a lot in it.
I much prefer the MkIV body size. The built in grip is much better than a body with a grip attached. And the weather sealing on the MkIV is much better
If I had to sell one of the bodies then I would sell the MKIV. The 5D is the better all round camera.
The MkIV is still a great camera and is no slouch when it comes to producing the goods. It was the best Canon had up to the release of the 1DX and 5D3 and will still blow any other canon
camera away.
Having said all that my MkIV will be up for sale soon as I fancy a 1DX
Gaz