I'd go for the Cartier BB but it's a no brainer for me bec. I don't care for Michelle watches and the only Chanel I like are the quilted flap bags with the chain, they look elegant and can go dressy. I just saw a rose gold BB at the supermaket on Fri. and I was in awe of it, such a beauty!
Also another thing to consider is that a watch stays on you and you can stare at it through out the day whereas a bag you just carry with you when you're out and about and then you put it somewhere when you go inside. I was actually deciding between an Hermes bolide, Hermes drag or a Cartier watch and I decided to go for the Cartier in the end bec. I would get more actual wearable use out of it.