420 mm. You can focus a lot closer. It is also surprising how much difference there is between 420 and 500 at times.
I have an older Nikon 1.4TC, and it works really well with the 300mm. With my 500mm, it is significantly less sharp with the TC. No idea why.
Thanks, Robin. The 300 with 1.4 TC is great for lager insects, butterflies, etc. You can focus pretty close and still stay far enough away to keep from frightening the subject.