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.