If you are considering the purchase of a Canon EOS 6D Mark II, you may also be considering the EOS 5D Mark III as they are similarly priced with an also-attractive, mid level feature set within the confines of today's camera market.
Make no mistake, these bodies are similarly versatile and capable cameras, but depending on the intended use and/or photographic challenges being pursued, either one may be a better choice compared to the other. With that said, let's look at how these full frame contenders differ.
Advantages of the EOS 6D Mark II over the 5D Mark III:
Advantages of the EOS 5D Mark III over the 6D Mark II:
As is evident above, the 6D Mark II has more advantage bullet points compared to the 5D Mark III. However, the importance of some of the 5D III bullet points could easily sway one's decision in favor of the older, economical 5-series body.
Who should opt for the EOS 6D Mark II?
First of all, anyone wanting to quickly create high quality videos with their DSLR will likely prefer the 6D's Movie Servo AF – thanks to its Dual Pixel CMOS AF sensor – as well as its vari-angle LCD. Another nice feature that DSLR filmmakers are likely to appreciate is the 6D II's ability to create 4K UHD time-lapses in-camera, though many filmmakers will prefer to compile their time-lapses in post-processing for more control over the final video (essentially making this feature a less compelling advantage compared to the 5D III).
From a video perspective, one drawback could be the 6D II's .MP4 recording format if someone instead preferred using the .MOV format featured in the 5D III. Another drawback is the 6D II's lack of a headphone jack.
Even though it features fewer overall points, those shooting wildlife will likely prefer the 6D II's slightly faster burst rate and AF system capable of up-to 27 active phase-detect AF points when using lens+extender combinations resulting in an f/8 effective maximum aperture. And on top of that, Live View with subject tracking can be utilized with lens+extender combinations through f/11.
In comparison, only the center AF point (with 4 assist points) is enabled on the 5D III with f/8 maximum apertures and subject tracking is unavailable in Live View. For wildlife photographers who never plan on using lens+extender combinations, the 5D III may be the better choice thanks to its 61 point AF system. But considering how often wildlife photographers utilize extenders, they will likely accept the 6D II's 45 point AF system to gain significantly more AF functionality at f/8.
Those carrying their cameras long distances and/or for long periods of time will of course appreciate the 6D II's smaller design and lighter weight, including (but not limited to) those who are hiking to remote locations or traveling on domestic/international flights.
All photographers will appreciate 6D II's higher resolution and longer battery life, and many will enjoy using its GPS, Wi-Fi, Bluetooth and NFC features which are all absent on the 5D III. Any photographers photographing under stadium lights will be thankful for the 6D II's Flicker Detection and corrective shutter timing for avoiding color balancing problems.
Who should opt for the EOS 5D Mark III?
Even though the 5D III seems to have few benefits over the 6D II, the value of some of those benefits can be huge. For instance, anyone photographing once-in-a-lifetime events such as weddings should likely choose the 5D Mark III for its dual memory card feature alone. While card corruption is relatively rare, dual memory card slots provide a vital layer of protection to keep your (or your client's) images safe. Having images backed up in-camera can help you avoid tarnishing your reputation due to a faulty memory card.
Many photographers will appreciate the 5D III's viewfinder with 100% coverage and faster maximum shutter speed.
If you already own a 7D Mark II, you will likely appreciate the very similar controls featured in the 5D III, including (but not limited to) the multi-controller joystick. If keeping the 7D II as a secondary camera, you should be able to switch between bodies without missing a beat.
Released in 2012, the 5D Mark III still remains relevant in today's camera landscape, although its target market has surely shifted from those needing cutting-edge technology (without stepping up to a 1-series camera) to an enthusiast group wanting full-frame image quality and a more-than-reasonable feature set at an attractive price. And, coincidentally enough, that's a good description of the 6D Mark II as well, although its overall feature set is certainly more contemporary.
As illustrated above, each of these cameras will serve specific photographers' needs better than the other. If you or your business can't afford the ill-effects of a memory card failure, or if the 5D III's controls make it a better fit for your photography, the 6D II's advantages will mean little when adding the 5D III to your shopping cart.
However, with its higher resolution, Dual Pixal CMOS AF sensor, more than sufficient AF system, GPS/wireless features and smaller design/lighter weight, the 6D II will likely be the preferred choice for a large number of photographers.