For me carrying the 300mm is similar to carrying the 70-200mm II. The 400mm being a bit shorter and lighter than the 300mm is very appealing.

Like the earlier thread about "crop factor", to me the equivalent framing might be about 20% or 480mm.

Most likely I will end up with a 300mm f/2.8 II, then the 800mm II if and when it comes out .