I'll say the other obvious thing ... The 400/5.6 is very portable and easily hand-holdable. The 300/2.8 isn't.
Ok, more scientific, here is a good comparison for IQ:
http://www.the-digital-picture.com/...CameraComp=9&SampleComp=0&FLIComp=1&APIComp=0
I think the 400 5.6 has it actually...
But then no doubt we will get into whether or not each lens in that test has been calibrated to the body used...
The 300/2.8 isn't.
Rubbish ( )! I hand hold mine most of the day!
Carl.