Well, you're comparing two different things. Bose QC15s are over ear (with noise cancelling circuitry), whereas the B&W C5s are in-ear (noise isolation).
Personally, I've been a fan of noise-cancelling headphones because most of them compromise the actual sound quality too much. But the QC15s are supposed to be pretty good in this regard (minimal impact).
In-ear headphones have almost complete isolation by nature and because they're pressure-fitted in your ear canal, they tend to have very tight and controlled dynamics. I've never heard the C5, but I'd also look into the VSONIC GR07 at that price range. I have a pair and they're quite good.