Headsets
I can only comment from the PPL point of view rather than ATPL but here goes. David Clarks are excellant at passive noise reduction (ie: without requiring power) but I've always felt like my head was being held in a vice after about 45 mins or so. I wear spectacles which doesn't help and the David Clarks are very durable but for me they don't suit.
For many years I've used Peltors which are quite light and comfortable and give adequate noise suppression. They have also proved to be very compatible when used with other makes of headsets, excepting a couple of issues with the aforementioned DCs.
We recently decided to go for ANR headsets (which have active noise cancellation) and narrowed our choice down to the Sennheiser or the Bose ones. The Sennheiser headset was very good, comfortable, light and with good passive noise attenuation (ie: when the battery's flat!), but the Bose were superbly comfortable, very light and in the end even though they were nearly twice the price this is what we bought. The only real snag is that the passive attenuation is not good at all so it pays to carry spare batteries - they last a good 20 or 30 hours though.
Hope this helps.