Well, if you are fortunate enough to walk into a boutique where there is a Birkin on display, I definitely recommend at least trying it on! The choice between Birkin or Kelly is a very personal one and it depends on many things: lifestyle, fashion preferences, whether you want/need a shoulder strap, your feelings about an open vs. closed bag...
And that's all assuming that you can find a Birkin in a size, color, leather and hardware you want. Kellys are much easier to find and unless you already have an SA who loves you and is willing to go the extra mile for you, buying a Kelly is a far less frustrating process in my personal opinion.
Good luck with your choice!