+1 on the low light and high ISO on the 6D. The amount of distortion (or lack there of) shooting in high ISO (> 8K) is really spectacular. The AF and the video capabilities are better on the 7Dmk2. Really depends on what you will be doing most (tank shots, outdoors, action/sports).
here is a head to head comparison from DigitalRev
Camera
7D MII 6D
Sensor Resolution
20.2 Million 20.2 Million
Native ISO Range
ISO 100-16,000 ISO 64 - 5,600
Continuous Shooting Speed
10FPS 4.5fps
Buffer Size
31 RAW 16 RAW
Max Shutter Speed
1/8000 1/4000
Movie Settings
1080p 24, 25, 30, 50 and 60fps 1080p 24, 25, 30 and 720p 50 and 60fps
Number of AF Points
65 Cross Type 11 Cross Type
Viewfinder Coverage
100% 97%
LCD Screen
3" 3"
WiFi
No Yes
Weight
910g 770g
Dimensions
149 x 112 x 78mm 145 x 111 x 71mm
Camera Build
The EOS 7D Mk II follows on from the tradition of the original 7D. It's built around a durable magnesium alloy body that has pro level weatherproofing to ensure you camera stays protected in the worst conditions. The improved weather sealing shows that Canon has thought a lot about this camera and how it is likely to be utilized for sports and wildlife photography.
The EOS 6D body is also built around a magnesium alloy shell like the 7D Mk II although the top plate is polycarbonate. In theory this may make the camera slightly less resistant to bumps and bashes but it's impossible to predict in the short term. Another small advantage of the 7D Mk II over the 6D is that it has a faster maximum shutter speed which is useful if you are shooting wide open in bright sunlight.
AF System
The EOS 7D Mk II comes with an impressive 65 cross type AF point system. This is the same system that is fitted to their flagship 1D X. Canon have also included their impressive dual pixel AF feature from the 70D which makes autofocus quick and smooth, great for video work.
The Canon 6D has a much less complex AF system. It only has 11 cross type AF points compared with the 65 of the EOS 7D Mk II. This will be perfectly suitable for any photography where you aren't excessively tracking your subjects like landscape or portrait work but if you are shooting motorsports for instance, you might find the 7D Mk II to be a little more accurate.
Speed
Again, when it comes to burst rate it is clear that the 7D Mk II is designed for sports, wildlife and action photographers. With an impressive 10fps burst mode, the 7D Mk II is a sports photographer's dream. The 6D on the other hand only shoots at a maximum of 4.5fps. For the majority of photographers this will be sufficient, even if sport is something you occasionally shoot. If however you are shooting sports regularly then you will much prefer the 7D Mk II.
Low Light
ISO range is an area where the EOS 6D pulls away from the 7D Mk II. As expected the 6D is a better low light performer due to the larger sensor and pixels that make its images much cleaner at high ISOs. The 7D Mk II has a decent ISO 100-16,000 range that is expandable up to 25,6000. The 6D however has an ISO range of 100-25,600 that is expandable both higher and lower to give a range of 50-102,800. In real use you are likely to see around a stop or so better low light performance from the 6D
Video
The Canon 6D's video features have been taken from the more expensive 5D Mk II. The 6D offers full 1080p HD recording at 24p, 25p and 30p. There is also the option to shoot slow motion 60p at a reduction resolution of 720p.
The newer tech inside the 7D Mk II has taken that one step further and now allows you to shoot 24p, 25p, 30p, 50p and 60p all at 1080p full HD. Combine this with the Dual Pixel CMOS AF system and you have a camera that is a slightly better all round video shooter.
Conclusion
It's important to understand that when we compare the EOS 7D Mk II with the EOS 6D that we are comparing two very different cameras. The EOS 7D Mk II is a camera that has clearly been designed for use by wildlife and sports photographers. The crop factor makes shooting with long focal lengths easier and the fast burst rate and AF mean that tracking subjects is a breeze. This all combined with the pro build quality means that photographers who are more used to using cameras such as the EOS 1DX will feel at home when using the 7D Mk II.
The EOS 6D is a different beast. It's a much simpler camera that is designed to get extremely high quality images in a relatively inexpensive body. It doesn't boast fast burst rates, fancy AF systems or a video spec that is anything special but what it does have is great full frame quality that gives you great high ISO performance, wider dynamic range and improved tonality. This camera would be perfect for a photographer shooting much more controlled subjects like a studio photographer or a landscape photographer for who speed offers no real tangible benefits.
It's also important to note the fact that the 7D Mk II has the advantage of being able to use crop sensor lenses which in most cases are slightly cheaper and smaller than their full frame counterparts. This is something to keep in mind if you are investing in the whole system of camera bodies and lenses.
To summarise, it's almost impossible to say one camera is better than another. In this case we are almost comparing apples to oranges. Saying that, if image quality is more important to you than speed of use and AF then you will find the 6D to be the best choice for your style of shooting. This camera is particularly suited to landscape and studio photographers looking for an entry into the world of full frame cameras. However, if you are more inclined to want fast AF, pro build quality and updated video specs but don't want the bulk or expense of the EOS 1DX or 5D Mk III then the 7D Mk II is the best canon crop sensor for sports and wildlife and will be ideal for you.
