This December I am traveling to Washington State to photograph bald eagles on the Skagit River. My main shooter is a D500. I’m considering renting an FX camera and a 500 or 600 f/4 and to use the D500 with my 70-200 f/2.8 for closer shots. So do I go for the speed machine D5 or the fine detail of the D850? I have no experience with either so I would appreciate learning from your experience. Thanks in advance.