A:
I enjoy the QC20 for flying more than the QC15 for a few reasons:
1) smaller and much easier to carry with you!
2) they have a mic, so you can use them as cellphone earphones and mic, so you don't only pull them out when you're on board and getting into your seat.
3) I felt noise cancelling was on par with the Qc15 in a plane.
However, for me, the QC20 has a more noticeable hiss than the QC15