<p>Let me start by saying I shoot with a Canon 6d and one of my friends shoot with a 5Dm2. We're both happy with our choices. As a 6D shooter, I've come to appreciate the geotagging capability for my landscape images. WIFI tethering is invaluable in extreme low light photography, as I can use my phone to adjust my exposure - "liveview" via smartphone. WIFI also offers the ability to get low angled shots that would be difficult to get on cameras without pop-out or articulating LCD screens. Finally, the 6d has its own sensor that is rated down to -3EV (able to detect enough light to focus in lower light scenarios than the 5Dm2 or the 5dm3).<br>
In either case, I'd submit that lens selection will ultimately determine how sharp your images are, and composition will ultimately determine how good your photos are.</p>