If you like reptiles that tend to be indoors, you want something that handles noise well and has a wide aperture and fast focus. Others have pointed out up and down the comments section, that usually the answer would start with lenses, and I agree. In your case, I'd probably get one decent prime lens with wide aperture (2.8 or better, ideally wider than 2) at the perfect focal length for your scaly friends, and then jump on the 6D.

I've owned a few 6Ds and I'm a huge fan. I'll take them anywhere. They're rugged and versatile. For your question about priorities, here are a few thoughts. If you like holding the shutter down for some serious FPS, then maybe that's important for you, but it hasn't been a high priority for me.

I DO like the bump up in number of focal points on the 6D mark ii, but in your case, it may not always be that useful, because the reptile exhibits will tend to be fairly visually busy, and you'll want to set your focal point on the reptile (or other critters) eyeballs, directly. The number of points are useful for letting the camera decide the optimal middle ground of scene. I'm not sure when this is ideal - maybe a family portrait of 250 reptiles, and you want to be fair? Or you're doing a wide-angle shot that takes in the entire orangutan exhibit, and none of them have earned the limelight of your focus.