I actually like the 6D better than the 5D3 since the 5D3 does not support interchangeable focus screens in the firmware; if you ever plan to use fast lenses the 5D3's lack of high precision screen will be a handicap in nailing focus. While the 5D3 does have superior autofocus, I am going to wait until 5D4 or 6D2 and hope one of them supports interchangeable focus screens like 6D... or I'll just buy 1DX!
One of the first things I was going to replace was my focussing screen for this reason. But in actual practice, I found I didn't miss the high precision screen because I can nail f1.2 on outer cross type points all day long, with total accuracy.
Now if you are using Zeiss or other MF lenses, yes, you will absolutely need to replace the screen for any sort of viewfinder accuracy at apertures wider than f2.8!