Kelly or Birkin, formal or casual style?

"The Kelly is a touch more formal, a little more appropriate for an evening out, a business dinner, as a more refined look. The Birkin is more sporty, more casual. Often people use it as a briefcase, throw in a change of shoes," says Trina Sams-Manning, manager of the Hermès shop in Fairfax Squar, Virginia (USA).
Kelly's bag was made for the elegant actress Grace Kelly and Birkin one for actress and singer Jane Birkin.  I prefer the Birkin bag even though it is heavy.
The price of these bags is about $6,000 or can top $25,000 depending on hide and hue.
And you, which do you prefer, the Birkin or the Kelly?