I have just been contemplating that particular choice, and I went for the 5D II.
What tipped it? - partly I think the fact that Canon rate the 5DII as 'Platinum' level camera and the 6D as a 'Silver' level camera for CPS membership.
I am not a member - dont have enough stuff yet (how do I put the sad smiley in here?), but to me it says quite a lot about what Canon think of these 2 in terms of professional use.....Just my 2 cents, as the guys across the pond say.
It seems 6D is on the same level as 5D3 and 5D2 at least on points...