While there may not be a huge difference between this model's noise-canceling prowess and the QuietComfort 2's, a distinction is definitely noticeable. According to Bose, these headphones are designed to defeat a wider range of frequencies, going up to over 90 decibels; the QuietComfort 2s peaked more in the 84-85db range. We also tried the QuietComfort 15s in the New York City subway system, and they did an impressive job of muffling noise.
Of course, the only problem is that because these are over-the-ear headphones that offer a tight seal, it can get a bit steamy inside the cups, especially on hot days, though they "breathe" fairly well for over-the-ear headphones. On long plane rides, your skin will also get a bit moist underneath the cushions, so expect to take them off for short periods to give your ears a little air. Also note that the 3.5 millimeter cord is detachable, so if you just want to block out the outside world and catch a nap, you can do so.
In terms of sound quality, the first thing we noticed about this model compared with the QuietComfort 2s was that the overall sound was smoother and more tonally balanced. In the QuietComfort 2s and QuietComfort 3s, Bose seemed to mess with the bass a bit, pumping it up (we found it a little thumpy), but the bass on the QuietComfort 15s is tighter and punchier (hip-hop fans may prefer the bass on the QuietComfort 2s, but that's a matter of listening taste).
The QuietComfort 15s offer excellent detail. Our first reaction to the headphones was an urge to relisten to our entire library of music, a welcoming symptom of acquiring high-grade cans. That said, these are a closed-cup design, so the sound is a bit more "stuck inside your head" than you'd get from open-backed headphones. As such, they don't quite measure up to some other high-end headphones that we've tested in this price range. Still, these are probably the best-sounding noise-canceling headphones we've heard to date.
As we've said about virtually all of Bose's headphones, they aren't exactly a bargain. And at $300, these aren't either. But they are a nice step up from the QuietComfort 2s and offer improved sound and noise-canceling performance. Looking at it that way, we can at least say they they're a better deal than their predecessors or the smaller and more expensive QuietComfort 3s, which still retail for $350.