Depends what your priorities are and what you want to spend. You could think of the 6D as a full frame 20D with a modern sensor and very few frills.
I have a 6D for a primary and a 50D for a backup body or for use with a telephoto lens when I want the reach. Looking to replace it with a 7D. I've owned a 7D before and high ISO really isn't one of its strengths, even relative to a 20D.
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