They are both such classic colors. It also depends the size, leather and hardware you are offered. If you truly think it will be your only birkin, it's also important to pick the right size for you (30 or 35 depending on your height/physique and what you want to use it for), and also whether you love the leather. And I like GHW with those colors so I would personally opt for that too. So many things to consider! I think every person gravitates more to one or the other of these classic colors. For me, I have very few browns in my wardrobe and a lot of colors and monochrome so black is a perfect neutral. It just depends which one you could see as your go-to bag. Good luck!
PERFECTLY Written! My sentiments exactly!