Whichever I already have lenses for. If I was buying brand new into a system and had no loyalty to either brand, I'd go with the Nikon D600. Since I do have loyalty/ experience with the Canon brand (I have never owned anything Nikon), I'd go with Mark II even if I was buying brand new into the system. But that's a personal preference.
HOWEVER, question for those that know: I'm not too familiar with Nikon products. But reading through the specifications, seems like the the D600 (And other FF Nikon cameras) has the ability to use in "DX" mode and thus use your crop-sensor lenses as well? Is this correct?