From amongst the 20 headphones in this series, the over-ear Bose QuietComfort 15 were easily the most comfortable. Slipping them on was a luxurious experience, capped off by the characteristic air seal that offers superb passive noise cancellation. And overall, they seem a little slicker. But unfortunately, that's where they peaked. The sound - only emitted with the battery-powered active noise cancellation turned on - was decent, but disappointing. The noticeable sibliance made for an unenjoyable audio experience, especially when listening to anything with S sounds or high hats.
The Audio-Technica ATH-ANC9 sounded kinda muffled when unpowered; but at least they weren't totally battery dependent, and that opened right up after switching them on. They also offered quite a different fit, with the padding right up against my ears. Still comfy, but took a little getting used to after Bose's sumptuous air buffer. On the flip side, the sound was much better: louder, bassier and less sibilant (although still hissy). I was able to detect subtle differences between its three active noise cancellations - indicated when wearing the headphones by different beeps and with them off by a little blue, red or green light on the outside - but all three of its modes (travel, office, study) seemed comparable to Bose at knocking out the low-end hum (such as you might hear on an airplane or from a clothes dryer). If I had to choose between them, I'd go with the Audio-Technicas.