Black. The argument about there being lots of fake Black Birkins is equally true for BJ Birkins, IMO, and you'd know that you're carrying the real deal.
MrsSparkles hit the nail on the head when she talked about dressing casually now and preferring a Raisin Birkin for her lifestyle. I wear my Black Birkin for business at least twice each week. Most of my suits and separates are black, so I like the sleekness of the Black Birkin and some color via an Hermes scarf. Or, I wear a Rouge H Box Calf HAC or a Gold Togo Birkin for contrast. Recently, I tried a Raisin Box Calf Maharana clutch when I was wearing typical work attire, and felt there wasn't enough contrast for it to pop. I preferred the Black Maharana as a "go with anything" choice.
So, black if you need a bag that will go with absolutely everything year-round, and raisin if you want something more casual. Finally, I hope you've seen these bags IRL or will have the opportunity to see them before you make your decision. The hardware might be what helps you decide. I'd prefer Raisin PHW, and Black GHW, but that's what would make MY heart sing. Only you know what's best for you.