If your budget is $6k, it seems a bit silly to settle for a crop camera. Lightweight and small are nice qualities in a camera system, but ultimately, if you're serious about photography, image quality is going to trump all that once you really get into it. That's just my opinion, though.
The Nikon D600 paired with a Nikon 14-24mm is probably the best value vs. quality proposition on the market right now for landscapes in general. However, for some types of night photography, the Canon 6D has a notable advantage in its cleaner images at very high ISO (6400-25600). One problem is that there are no ultra-wide lenses for EF mount that can equal the Nikon 14-24mm. I use the Samyang 14mm on my 6D, but its severe vignetting in the extreme corners is very problematic if you want any decent foreground detail on a moonless night. It is possible to get an adapter for the 14-24mm and mount it to a Canon DSLR, if you decide the high ISO noise advantage is compelling enough to go for the 6D.
Bottom line: if you plan to shoot a lot of daytime landscapes, and night photography will be more of a side thing and focused primarily on auroras, then the D600/14-24 combo is unbeatable. You'd still have about $2k left over to play around with mid-range and/or telephoto lens options (unfortunately, when it comes to Nikon's own branded lenses, it might be difficult to find both for that price). If night photography is your main interest, though, and you want to get into shooting star trails, nightscapes and the milky way even when auroras aren't happening, the 6D is worth a look.