I can tell you that when it comes to birds, more focal length is better in almost all cases. Getting closer is better as well. In this case you will notice that even though you had to crop a little the image quality did not suffer....that's what more focal length and getting close will do!
I think what it boils down to is that the longer focal length combined with close proximity gets you more pixels on the subject and hence better IQ. I would love to have the version 600mm. I think that lens with the option of using teleconverters is probably the best and most verstile kit for bird photography that you could have....it's very pricey however![]()