Although it offers the same magnification (and therefore physical size), the 5D IV delivers a what-you-see-is-what-you-get 100 per cent field of view - whereas the 6D II's 98 per cent FoV means the outermost edge of the frame won't be seen in preview, only in capture. If you're non-plussed about a vari-angle screen because your eye will be almost permenantly fixed to the viewfinder then, well, the 5D IV is the better of the two.
The 6D II is slightly smaller and lighter than the 5D IV, but not by a giant amount - plus you'll barely notice one adding a lens. Both are full-frame, both accept Canon's EF lens range, both are water- and dust-resistant too. Objectively, the two cameras are fairly similar.
(Pocket-lint) - When the Canon EOS 5D MkIV burst onto the pro camera scene in 2016, it hit so many tick boxes that, well, few other full-frame cameras seemed to be worth a look-in.