Its like comparing the 28-70 f/2.8 and the 24-70 f/2.8.
Sure the 24-70 is newer, more recent technology, but is it really better?!
My 5D Mk II makes me perfectly happy, the only thing I would upgrade to is the MK III.
The 6D is a good camera for sure, but (as my friend also said) its not that better, that its wort to "upgrade".
If you are coming from a 7D tough, the AF system of both these cameras is worse then that of the 7D.
AND THAT could make you being disappointed. If you really want to feel like upgrading, then the 5D Mk III is the only way to go, it is the only (not 1 Series) camera realy superior to the 7D.
Now if you can't afford the 5D Mk III, and looking at the prizes, the 5D Mk II should be around $2100, and the 6D around $2600.
The question is: Is the 6D that much better for your needs, that is it worth the $500 extra, or can you get something for the 5D Mk II for $500, that makes it a better deal.
In all honesty, comparing camera specs side by side does not make that much difference.
Its the photograper, and his (rest) gear that makes that difference.
For example, for that $500 you could get a nice tripod, and use it to make nice night shots...
Now its nice that the 6D has better HIGH ISO performance, but without that Tripod, it does not really matter, because you won't shoot night pictures at 1/20sec, but at 30s, and for that you NEED a tripod, or something that holds your camera.
Or you can get a quality ND filter, and make stunning long exposure shots, at daylight - without that filter, no matter wich camera is better, you can't make these shots.
(Just my opinion)