I have used the Bose QuietComfort QC-2 headphones for about a year, and have traveled to Asia over 5 times with them, and domestic travel as well. About 120 hours of air time. Never took them off. Vastly reduces my jetlag. A pleasure to own and use.
On an impulse buy, I bought the QC-3's after a demo at the local Bose store. 30-day return policy. Compared them side-by-side... both on the airplane, home, with and without sound.
My impressions (your mileage may vary :-)
The QC-3 has impressive noise cancellation, I think it's incrementally better than the QC-2. Perhaps it's a little too 'aggressive'. Let me explain. I feel a little uncomfortable from a (perceived) pressure on my ear. Note: this is not physical pressure from the ear-pieces, nor is it air-pressure, but rather a physco-acoustic result of noise cancellation. I felt this only slightly with the QC-2's. Bose did an amazing job of noise cancellation given the challenges of an "on-the-ear" design which offers less physical noise isolation due to lack of cups. Everyone has a different tolerance to this pressure, the only way to tell is to try them out. Vote: QC-3 for noise cancellation, QC-2 for comfort (again, will vary from person to person).
Using MP3's encoded at 192 kbps, and CD audio, the difference in the two headphones is clear (excuse the pun). The QC-2's are brighter sounding with adequate bass. The QC-3's are a little heavy on the low-end, to the point of sounding muddy. Vote: QC-2.
Some have complained about the QC-2's breaking at the stem that attaches each earpiece. I never had this problem, but I am also very careful, especially given their cost. I did notice a person break a QC-2 on the plane trying to turn the earpieces to hard. The build quality of the QC-3 is about the same, the stem may be somewhat narrower, so they could be more prone to breaking, however I do not think it's an issue if you treat them with respect. Vote: equal.
The QC-3's are lighter, but I still prefer the "over-the-ear" type of headphone. They feel like they stay in place better. I would be upset if the QC-3's fell off my head while loading luggage in the overhead bin, only to have someone step on them. I never felt that could happen with the QC-2's. I also would rather have a little physical pressure around my ear, instead of on my ear. Vote: QC-2.
The QC-3 has a less complicated jack that attaches to the headphone (it's detachable, just like the QC-2's). The QC-2 has a hi/low volume control on the jack stem, while the QC-3 has no adjustment and is adjusted somewhere in between (which is why they are not quite as loud as the QC-2's, given the same volume setting from your MP3, DVD, computer, or stereo). The airplance adapter does serve to attenuate the audio further, if needed. The QC-3 carrying case is a little smaller than the QC-2, but not a significant difference. The covering on the QC-3 case is smooth, which I prefer over the QC-2 case. I do not like the way the QC-3's fit in the case, the ear-pieces do not sit firmly where they are supposed to go. With the QC-2, you know exactly how they fit in the case, it just seems like a better fit. Vote: averages to equal.
I hope this quick review is helpful. I elected to stay with the QC-2. Rgds,