I have the 300mm prime f/4 IS L lens and am very happy with it. Combined with the 1.4 converter and the 1D it is nice combination buuuuut as is the case when chasing small birds a little longer reach would be great

Anyways I have been considering buying the 400mm f5.6 or the 100-400mm zoom (while I would love something longer at f/4 or f/2.8 my pocket money does not stretch that far).

Two questions:

Is either lens better/sharper than the 300 combo??

Which is the preferred lens from the two above.

I appreciate any comment pro or con that might help me. Images taken with either might help too.