The first major difference is with the screen. The 5D IV has the larger and higher-resolution panel, but the 6D II is the first full-frame Canon DSLR to offer a vari-angle touchscreen.

Canon 5D Mark IV vs Canon 5D Mark III at Base ISO. Canon 5D Mark IV at ISO 100. Canon 6D Mark II. Canon 77D. Canon 7D Mark II. Canon 90D. Canon G5X Mark II. Canon G7X Mark II. Canon G9X Mark II.
Thanks, everyone, for your input. I've decided to get a Canon 5D Mark IV. I find that the main things I am lacking in my present set of gear are very low light capability and weather resistance. I think that the 5D Mark IV best fills these wants. When I need to photograph the night sky, I will still have my 6D since I don't plan to sell it. This new R6 Mark II runs faster (40 vs. 30 FPS) than the R3 with tracking focus and exposure! The clumsy Nikon Z9 can go faster, but only at 11 megapixels. Worse, the Nikon Z9 only runs at 30 FPS with a lot of limitations, while the R6 Mark II runs at 40 FPS all day with no restrictions.
If they need dual card slots the 5D Mark IV is the better choice. Also the 5D Mark IV offers 4K video over 1080p video that the 6D Mark II offers. But I think OP would be better off with the 6D Mark II. -Demetrius. 40D, 5D IV, EF 16-35mm F/2.8L III, EF 24-70mm F/2.8L II, EF 28-135mm F/3.5-5.6 IS USM, EF 50 F/1.8 STM. 430EX III-RT & 600EX II-RT
. 248 217 467 478 440 287 164 210

canon 5d mark iv vs 6d ii