Ergonomics matter a lot.
Then, what do you shoot? If you are a landscape and macro shooter, you won't care about the 6D having a measly 11 AF points because you don't use AF all that much. I use mine with some film-era legacy lenses as well as some modern AF-capable lenses (one of four lenses I took on Saturday's outing had AF, the others were Zeiss, Samyang, and an ancient Nikkor AIS). If you are a sports shooter, ignore the 6D. For action, you will want the 5D3, D750 for full frame non-pro-gripped cameras. If you are looking to be a budget birder, I would go for the new Canon 7D2 APS-C camera with the old Canon EF 400mm f/5.6L non-IS and Canon 1.4x teleconverter II or III (greater pixel density on APS-C gives you more resolution for any given focal length).
I am a Canon shooter, and by and large the lens types offered by Canon and Nikon are similar, with some exceptions:
Canon specialties: 400mm f/5.6L, best prime budget birding lens out there, $1350.00 USD and 1.25 kg. MP-E 65mm f/2.8 1x to 5x macro lens, like having a bellows set-up, but handles much better and is suited to field use - every expert insect photographer seems to have this lens. And, these tilt-shift offerings from Canon: TS-E 17mm and TS-E 24mm. Nikon has longer TS focal lengths (called PC by Nikon), but not the short FL. Oddball, not sure if there is a Nikon equivalent: zoom fisheye 8mm to 15mm.
Nikon: the fabulous 14-24mm f/2.8 lens. Canonista landscape photographers have bought adapters and use this lens on manual-everything mode. Giant brick of a lens, needs special filter holder for 150mm to 165mm sized filters. By the way, Canon can use Nikon lenses with adapters, Nikon cannot use Canon lenses, due to different lens flange to sensor distances.
Sensors: Nikon is the leader here, without a doubt, due to the Sony sensors. Better dynamic range, more MPs. Canon is "good enough" but not great. Still, the 6D is a fine low light camera.
The Canikons of the world are huge compared with the Olympus and mirrorless offerings. Ergonomics is huge here, some like the little cameras, some like the big cameras, some people swear by optical viewfinders, some like electronic viewfinders.