The main difference is one being an f4 lens and one being an f6.3. The standard for professional wildlife photography has always been a 600mm f4. There's nothing wrong with the f6.3, but most wildlife comes out early morning or near sunset, when there isn't much available light. You'll be pushing iso pretty far with the f6.3 lens. If shooting in the middle of the day the f6.3 lens has the benefit of being the lightest 600mm lens in production.

What are the key differences between the Nikon Z 600mm f/6.3 and the Nikon Z 600mm f/4, and is the f/6.3 version a good option for wildlife photography?