I would go with the black B-bag. It's a timeless, seasonless bag that is super lightweight and functional. I love mine!
I vote for the B-bag also, you won't be disappointed....in fact you'll get addicted! If you do decide on the YSL Muse go for the buffalo/leather instead of the patent....I think the patent dates it AND gets marked up easily, not to mention you got to constantly keep it clean and shiney!:s
The two bags are so different in style and size that its hard to say without knowing your needs and what else you already have...BUT, I would agree with pursemama that the YSL Muse in the regular buffalo leather, not the patent, would probably be more timeless.
Mmm.. I like both but I completely agree with not getting the Muse in Patent Leather. If I had to pick my vote would be for the Balenciaga.
Ditto - of your choices, I'd go with the Bbag. The patent will be really dated again by the end of the winter season.