Why was Lily's sacrifice the important one?

[sugaranddixie1]

No: HPFGUIDX 149910

ET-
In other words, why does her giving her life for Harry turn out to be 
be what protects him?  James also sacrificed his life to try to protect 
both Lily and Harry. So why the almost total emphasis on Lily? The only 
thing I can come up with is that it had something to do with 
the "purity" of Lily's character- her ability to see good in anyone...
Or was it because she was given the choice of stepping aside & didn't 
take it?  Did her making that choice make it the ultimate sacrifice?   
As far as I can tell, James wasn't offered that sort of chance....  Any 
thoughts?

sugaranddixie1