I compared the two entry-level FF cameras available from both manufacturers at that point in time, namely Canon 6D and Nikon D610. Price-wise, around the same. But I ultimately chose the Nikon because it has more features that I wanted, namely:
- 100% viewfinder coverage
- 39 AF points with 9 being cross-type sensor
- 2 SD card slots
- Built-in flash
Canon 6D does have some advantages over Nikon D610; for instance, built-in Wi-Fi and GPS. I don't really care much for built-in Wi-Fi, but GPS would be useful for me as I like to geotag my photos.
Moreover, the 39 AF points of Nikon D610 sounds awesome compared to Canon 6D's 11, but they are all clustered in the centre. reducing their usefulness.
I also heard from more experienced photographers that Canon is better in video than Nikon. Since I am not into video (yet), that is irrelevant to me. I did recommend a friend the Canon 6D because of this though, as he is more into video than me.
To conclude, if you are an existing Canon or Nikon cropped sensor camera user and thinking of upgrading to FF, I would recommend sticking to your existing system unless you really know what you are doing. However, if you are coming fresh into the FF world and thinking of choosing either a Canon or Nikon entry-level FF, then Nikon D610 would be the safer bet.