it all about personal taste. I am more into dark colors, MBB is awesome color, but I preferred to go Tanzanite. Though as you said tanzanite needs to be clean to sparkle otherwise it is a dark blue, which is ok for my liking. In fact in my case, I was between carbon black and tanzanite
I am attaching sun vs shade to get an idea
Also one MBB VS TANZ side by side (cloudy day)