Comfort

Perhaps my ears are more sensitive than many people's.  I don't like sticking ear-plugs into them, and similarly I don't like using a set of 'on the ear' headphones, I prefer to use 'around the ear' headphones.

The around the ear style press onto your skull and form a seal against the skin on your skull.  The on your ear style press onto your ear and form a seal against the outside of your ear.  It seems the seal is better around the more evenly shaped skull, and there's less sensation of pressure on the skin/skull then on the cartilage of your ear.

Comfort is a very important issue, particularly if you're going to be using the headphones for a long flight, maybe as long as ten or fifteen hours.

My personal preference, from the comfort issue, is for the around the ear style of headphones, either the Bose Quiet Comfort 2 or the Plane Quiet Solitude headphones.  On the ear headphones are definitely smaller, but in our opinion are not as comfortable to wear for extended periods of many hours at a time while on a long flight.

You probably know your own thoughts about this issue, and so can guess which style is more likely to be your choice.

The ear pieces have very soft padding on them to make them as comfortable as possible and to mould to the shape of your ear as closely as possible.  This helps to passively block out sound.

Unlike the QC2, the QC3 doesn't have a level selector switch built into its connector cable.  This is actually a relief - the level selector switch on the QC2 was in a very inconvenient place (you had to unplug the cable to access it), and with improved electronics and no need for a level selector switch the headphones are more convenient in use.

But, and like the QC2, the QC3 is gratuitously designed to force you to buy only Bose connecting cables.  Sooner or later, the connecting cable is sure to wear out (or simply to be lost) and Bose seem to have deliberately designed the connecting cable to be of non-standard size and type at the end that plugs into the headphones, preventing you from simply buying a replacement cable for $5 from Radio Shack.  Instead, you must pay $15 plus shipping from Bose, and if you lose/break a cable while you're traveling, you're pretty much stuck without a replacement until you can get one shipped to wherever you are.  This is a bad example of Bose choosing to put their greed ahead of our convenience.