1) The D600 has the far better AF system.
You are absolutely right, but the question was for a digital, 35mm OM replacement.
2) I do not see much sense shooting old OM lenses on a 24 (or 22) MP camera, they for sure are not up to the needs of this resolution
Correct also, but there is no current 35mm DSLR available that will mount the OM lenses
and have less megapixels.
3) Using GPS and WiFi significantly reduces battery life, so when you want to shoot only close to home or in studio that does not matter ....
I'm doing a lot of travel photography, and stock agencies have started asking for coordinates. GPS would save me a lot of boring work during post processing. WiFi... I don't need that, but I suppose it doesn't gobble up batteries if it's switched off.
4) Not sure why you say the 6D has better video, I would assume the other way around
I saw a test somewhere, can't remember which website. Not important for me. A GH3 will eat them both for breakfast anyway with its superior functionality for video.
5) higher ISO is same or better with the D600 - do not believe all what is written in Internet published tests
This, I've seen documented a number of times. Interestingly, the 6D shows less noise than the 5DIII at high ISO as well, but at the cost of less detail.
6) if you have Nikon (lenses) already why mess up with Canon else you really want to go for some of their high speed primes, which are not available from Nikon
Again, it would be the digital OM, digital Contax etc.
To make it short, for the 6D I would not jump ships, maybe only for a 5DIII with some high speed primes
Oh, but I am jumping ship... to m4/3. The 6D for me is rather hypothetical at the moment, and if I'm buying a camera with a 35mm sensor, the D600 would certainly be first on the list. But at the moment, I'm too busy buying m4/3 lenses and then there's the GH3