The birkin will fit more in and is a much larger bag. But the Kelly 20 is super popular right now!Surely you request what you want most
neither will be easy to source
Do you want a very small Kelly, with all the difficulties of access
or do you want a 'shove your hand in the opening' Birkin with easier access
as you have so many Bs and Ks, have you considered a Constance?
They are all so different.