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Ok now I have to get out my 1D3 to check the statement that the 6D has a brighter viewfinder. I'm not sure that it does.
The 1D3 offers 100% viewfinder coverage, a much better AF system, and the image quality is pretty darn good, even if it has only 10 megapixels. I've printed 1D3 photos at large sizes and been plenty pleased with the results.
It shoots at 10 fps, has 19 cross-type AF sensors with 26 assist points, offers great battery life, and AF-point Linked Spot metering.
There is a lot to like about the 1D3.
All the merits you note on the 1D3 are true and the reason I won't be getting rid of my 1D3. I still prefer it for wildlife shooting but just about everything else, I now go to my 6D. Which in my opinion, has the brighter viewfinder and better image quality.