I have a 5D mark II and love it. However if i were buying right now i would buy the 6D for all the reasons stated above. The 5D is more rugged and weatherized, but I baby my gear, so it doesnt need to be as rugged as it is..... Plus you can always buy a rain jacket for the 6D if you think things may get a bit dicey.
They are both great cameras and will take beautiful pictures, the 6D is just going to give you more range (less noise) on the darker end.
And you won't regret it, it's a great camera. I nearly bought canon when I started out but, and this sounds stupid, I hate the grey color of their top lenses. Just makes them look cheap and flashy in my mind.