Sorry, but I have to disagree...
5D Mark III is a 6-1/2 year old model. 6D Mark II is a little over 1-1/4 years old.
Both are full frame...
Sure, the 5DIII has more AF points, 61 total. But of those 41 are higher performance "cross type" (the rest are lower performance, single axis type). One of the camera's AF points is f/8 capable (center point) and the AF system is able to focus as low as -1EV. ("f/8 capable" allows more lens/teleconverter combos to be used.)
6D Mark II uses a 45 point AF system (intro'd in 80D about 2-1/2 yrs ago)... all 45 of which are "cross type", up to 27 of which are f/8 capable (varies depending upon lens), and it's able to focus as low as -3EV light levels (approx. "moonlight"). The 6D Mark II's AF was significantly upgraded from the original 6D's. Same with the 5D III's, which was greatly improved over the 5D II's and original 5D's.
However, between the two of them, the 6D II's AF system is a generation or two newer and a bit more capable in several ways than the 5D III's.
6D II has Dual Pixel AF in Live View, which is far faster than the contrast detection the 5D III uses.
6D II has nearly 20% greater resolution (26MP vs 22MP).
6D II has slightly faster frame rate (6.5 fps vs 6 fps) and a bigger buffer (98 JPGs vs 63).
6D II offers higher native ISO (ISO 100-40000 vs 100-25600... both expand to 50-102400)
6D II has a bit more usable high ISO, a little greater dynamic range, greater color depth, and overall higher rating by DXO.
6D II has a fully articulated 3" Touchscreen LCD... might be nice for landscape photography (5D III's is 3.2" fixed & not Touch... both are 1 million pixel).
6D II is lighter weight and is rated to get nearly 33% more shots per battery charge (both use LP-E6/E6N batteries).
6D II has built in wireless connectivity and GPS, which might be nice for landscape work, among other things. 5D III has neither (avail. using optional, separate modules such as WFT-E7/E7A).
On the other hand...
5D III has higher spec shutter.... top speed of 1/8000 (versus 1/4000) and flash sync 1/200 (versus 1/180).
5D III might have higher durability rating.... 150,000 "clicks" versus 100,000. Not sure about this, though.
5D III has dual memory card slots (one Compact Flash & one SD versus a single SD slot in 6D II).
5D III has 100% viewfinder (versus 98% in 6D II).
5D III has a PC sync socket (for wired connection to studio lighting... 6D II doesn't).
5D III has a headphone jack socket (6D II doesn't).
Don't get me wrong... 5D Mark III is a fine camera and very capable. If I already had a 5D III, I think there would be little reason to "upgrade" to 6D II. The differences just aren't that great. But when you have neither and are weighing a choice between them, the five year newer 6D Mark II matches or is incrementally better than 5D III in a number of ways. The 5D III only out-performs the 6D II in a few ways.
Also, the 6D II is currently in production and fully supported by Canon. The 5D III was discontinued in mid-2016 and eventually Canon will stop supporting it and run out of spare parts to make repairs to it. This happens sooner or later with all models, of course. But it's likely to happen sooner with an already-discontinued model, than with one that's still in production (and probably will be for around 3 more years, if we use the production lifespan of the original 6D model as a guide). https://cameradecision.com/compare/Canon-EOS-5D-Mark-III-vs-Canon-EOS-6D-Mark-IIhttps://www.imaging-resource.com/cameras/canon/5d-mark-iii/vs/canon/6d-mark-ii/https://www.apotelyt.com/compare-camera/canon-6d-mark-ii-vs-canon-5d-mark-iiihttps://www.dxomark.com/Cameras/Compare/Side-by-side/Canon-EOS-6D-Mark-II-versus-Canon-EOS-5D-Mark-III___1170_795https://www.bhphotovideo.com/c/compare/Canon_6D_Mark_II_vs_Canon_5D_Mark_III_vs_Canon_5D_Mark_IV_vs_Canon_70D/BHitems/1346734-REG_847545-REG_1274705-REG_986391-REG