Disclaimer: I only own a 7D, and (for my personal needs/wants) I will soon be upgrading to a 7Dii.
I have heard from several people I know that the 6D is a fantastic body for low-light landscape and starscape photography. The IQ is excellent, and the dynamic range is quite good as well.
I've personally used my 7D to very good effect for studio and location portrait work. While there may be benefits inherent to FF sensors for this type of work, I have not personally felt limited by the 7D in this regard.
I have used my 7D for quite a bit of low-light, high-action photography (high school football/basketball, concerts, etc.). This is where it struggles a little. It's not great at the higher ISOs needed for these venues. The camera HAS limited me in these areas - given that I'm shooting good lenses with larger apertures and still bumping up against (and exceeding) the limits of the camera.
So, if you primarily want to expand on your landscape work, it sounds like you should go with a 6D and keep your 7D for action photography - assuming you don't need the low-light capabilities.
I'm going with the 7Dii because of the low-light focusing and better high ISO performance. I suppose I would prefer the 5d mkiii (or 1Dx - yeah right!!) if money were no object. It would simply cost too much for me to make the shift in terms of lenses at this point. That, and the 7D mkii has a host of other features and attributes that make it a pretty easy decision for me at this point.
Canon 7D Gripped; Canon 7D ii Gripped; Canon 17-55 f/2.8 IS USM; Canon 70-200 f/2.8 L; Canon TS-E 24 f/3.5 L; Sigma 85 f/1.4; Rokinon 8mm f/3.5; various lights & gizmos
Website: www.jsvphotography.com