I have complete faith in the 6d's capacities in landscape, low-light etc. but my primary interests have more to do with how skin tones come-out as well as how much weight the profile of the person has in relation to the background etc.
I can't speak to the artistic qualities directly, as I am not a portrait photographer. From a technical and technological standpoint, however, I would offer that the 6D's IQ should be just as good, if not better, than the 5D II. It is a newer sensor, generated with a more refined process (not necessarily new process...it is still the same old 500nm process that Canon has used for over a decade, but it certainly more refined.)
On the standpoint of color reproduction....that is really all just math. The "strait out of the camera" quality can indeed be influenced by the sensor design and manufacture, and on that front, my previous point holds true. It is the same process, same general technology, with a number of refinements and other improvements. The rest is all math...tone curves applied per-channel during the demosaicing process. Technically speaking, there should be nothing, whatsoever, stopping you from achieving the tonality you desire. You could even pick up Adobe's camera profile generator for Lightroom, use a Color Checker card, and generate profiles that best fulfill your expectations. Set one of those as a default, and every time you import photos into Lightroom, your custom profile will be applied, accurately reproducing YOUR expectations.
As for the subject relative to the background...does that not all boil down to framing and lighting? The 6D is a full-frame sensor, so technically speaking it is no different than the 5D or 5D II from a perspective, DOF, and boke standpoint. If you use an 85mm lens up close, with a distant background, you'll get the same composition and perspective out of the 6D as you would out of the 5D, 5D II, or any other 36x24mm FF sensor. Any other factor that relates subject to background would have to do with environment, rather than equipment. If you want your subject to be isolated against a dark background, do the same thing you always would...subject close to the light source, background far from the light source. Etc. etc.