The image resolution of the 5D MIII (22MP) and the 6D MII (26MP) is very comparable and they feature the same exact sensor size (Full frame). The 5D MIII was launched 6 years earlier than the 6D MII which is a fairly big gap as far as camera tech is concerned. Both the cameras offer the identical body type (Mid-size SLR).
Canon EOS 6D vs. Canon EOS 5D Mark III. The Canon EOS 6D and 5D Mark III are both previous-generation models that can be found at good prices on the used market. Which is a better fit