400mm f/2.8 or 600mm f/4 - Which is the right super telephoto lens for you?

I have photographed mammals and birds with both lenses and would like to help you with your purchase decision. You can buy both lenses and accessories here (affiliate links): Canon RF 600mm f/4 L IS https://bhpho.to/48YNuHT Canon RF 400mm f/2.8 L IS https://adorama.rfvk.net/jrxqnP Canon RF 1.4x Extender https://bhpho.to/3RW1cWj Canon RF 2x Extender https://bhpho.to/3GY9YNs Tripod Foot https://bhpho.to/3vGlvyt LensCoat RF400/2.8 https://bhpho.to/4bRiew6 LensCoat RF600/4 https://bhpho.to/4buaMXO