Personally for my Speedy B I purchased recently I went with Damier Ebene. Simply because for a speedy shaped bag it's marginally (although only slightly) less common and less faked, in my opinion.
It's also the original fabric, monogram isn't, and in terms of "iconography" or "classic" bags I think a "classic" bag such as the speedy looks best in a "classic" (AKA original) fabric.
It also requires less upkeep which appealed to me, as the leather doesn't patina or anything it's easy to carry every day which is nice.
Obviously these are all my personal opinion, I don't dislike monogram, I think it's a great pattern and I would consider purchasing monogram in the future I just personally prefer Damier as a whole.