600mm f/4 or 400mm f/2.8? Lens buying thoughts from a wildlife photographer

The first 1,000 people to use this link will get a 1 month free trial of Skillshare: https://skl.sh/stevemattheis07211 Joel's instagram: https://www.instagram.com/joelcbrown/?hl=en Buying a lens? Please use my affiliate links in the 'Gear I use' page below: https://www.stevemattheis.com/gear Here are direct links to the two lenses in the video: Sony 400mm f/2.8 https://alnk.to/dAgkwab Nikon 600mm f/4 https://alnk.to/eZAoIUP or https://amzn.to/2Ks4LxI