You sir has what I like to call rich people problems.
Its like asking what should I buy a Mercedes or BMW.
I cant afford either so I don't have your dilemma problem (wish I did).
Here are my thoughts and important points
1.Overall according to reviews I read and saw the D600 is the better camera
2.6D low light performance is amazing!!!
3.While the 6D low light performance is better the D600 low light performance are very, very good!!! probably more then what I will ever need for most occasions.
4.6D has no flash
5.D600 has in few of its cameras dust oil spec problems
6.As said by others D600 Dynamic Range is better then 6D
7.D600 has better AF system
You really cant go wrong with either.
If not for the oil/dust issue in very few of the D600 I would say with no hesitation go with the D600, that's the only thing that slightly bothers me.
This issue is really blown out of proportions, you really can clean the sensor yourself at home and after few thousand clicks it goes away and that in case you encounter the problem in the first place.
Again most D600 users don't have this dust/oil problem but it is important to add this to your overall argument pro and con with these cameras.
Good luck
