For full frame look at the systems that you are buying into. Right now, nothing compares to Nikon or Canon, so i would stick to those two unless you have compelling reasons eg you're a Leica snob. Now my pet peeve - evf - hate it - nothing like looking through an optical viewfinder. BTW f2.8zooms are expensive and heavy. Canon has a 24-70f4L that is fantastic lens if you don't really need f2.8. Big difference between the 5dIII and the 6D, I would not get the 6D. Plus you can now get the 5DIII in 3 flavours.
Nikon sensors have surpassed Canon's in terms of dynamic range captured. Their AF and flash systems have always been the best out there. However, I find the Canon lens lineup to be more extensive and slightly cheaper across the board. Plus Canon does IS really well.