The following is something interesting from Romy O'con at Pbase comparing the quality of lenses he owns:
http://www.pbase.com/liquidstone/300_vs_400
It seems that he uses the F5.6 400mm often for hand-held bird-in-flight shooting and along with the tripod when that lens is coupled with a 1.4x and 2x.
I prefer the 400mm for its manageable weight and the definition the glass of this lens can offer. Perhaps, weight is a factor that weighs high to your daughter when making a decision.
Peter
