A week ago, I had the pleasure of testing the Sony WH-1000XM6 in the DA40 (LN-NEZ), which we have a tiny (read super tiny) share in.
I have previously tested the XM6 as a passenger in a noisy 777 and was then somewhat stunned by the quality of the noise cancellation in the XM6.
‘Our’ DA-40 LN-NEZ has been going through a midlife upgrade since early this year, and has not been available for flying till now. 
The midlife upgrade gave it a new engine (from 135hp to 155hp diesel), new (2x) radios, new digital instruments, all LED lights, new paint, plus several non-visible upgrades. All at a cost of around EUR 190k…

The audio test
For the test of the XM6, we used the Bose QC45 to compare.
There is simply a difference both in comfort and in the level of noise cancellation.

• The basic comfort of these two headsets is comparable, but when using glasses, the XM6 feels much better on my ears.  
  This can be caused by the fact that the earpads on the Sonys are slightly larger than the earpads on the Bose (any Bose in fact). 
  The earpad is resting on the skull – and not on the ear, and I do believe that makes a difference.
  
  And for us men, the ears are sadly one of the very few body parts that are growing (apart from belly, nose etc etc).
  
  
• The sound quality on the Bose and the Sony is, for all practical purposes, comparable, but the noise cancellation on the Sony is really incredible.
  When switching headsets in flight, it is very easy to hear that the noise cancellation in the Sony is SO much better: easy to hear in the fairly noisy DA-40 (now with the 155hp engine).
  And for your info, both left and right seat agreed on this (the reduced noise level with the XM6- not the upgraded engine!).
  .

Ease of use
The Bose headsets have always been easy to use, but with the introduction of the QC45 series (including the SE, SC , Quietcomfort and numerous names for the same product), Bose added a volume control on the headset. And with the volume control, they also added the feature that the volume went to minimum if the headset was switched off or unplugged, with the Avee (or the 2.5mm connector) connected.
Clearly a bug but it is annoying and for sure not needed.

The Sony does not have this annoying feature: One button to turn on and the same button to turn off.

Both the Sony and the Bose work even when the battery in the headset is flat. Battery capacity is comparable; maybe a few hours better on the Sony.

Build quality and Avee implementation
The build quality of both the Bose and the Sony has always been excellent.
Our general feeling is that the build quality on Bose headphones is going down, while Sony has chosen to go in the opposite direction with the build quality.
The earpads on the Bose tend to wear out fairly quickly, and they are more than a pain to replace (see below).

The Bose headsets have always used a fairly fragile 2.5mm connector for external connections, while Sony use more sturdy 3.5mm connection, which is simply a better choice.

The Avee looks well on the Bose, but looks even better on the Sony!

Changing the earpads 
After using the Bose headphones for a year or so on a fairly regular basis, you will need to replace the earpads. And this is something you can look forward to: it is a real pain. Simply because the build quality of the pads and the headset is not really good.
Finding good replacement pads can also be a real pain.

Changing the earpads on the Sony however, is a totally different story. You do it in less than one minute (with no practise apart from a YouTube video) and the pads that are normally available from the same store where you buy the XM6, are Sony-made and of excellent quality! Way to go Sony!

General use as recreational headsets
Both headsets work well as recreational headsets.
The Sony XM6 is more advanced because it supports Bluetooth Low Energy Audio, (BLE) but it is early days for BLE. Getting back to this. Excellent audio quality for recreational purposes on both headsets. 

Price level
You would expect that the price level on the XM6 is higher than on the Bose QC45 series, and this was correct some months ago.
Here in Norway, we would pay something similar for both the Bose QC45-series and the Sony XM6 (around EUR 350 + VAT).

Final words
If you do not have a headset yet – or want the best for your ears- go for the Sony XM6. It is a better choice than the Bose.
If you already have a functional Bose, wait till the earpads wear out or until the headset fails: then go for a Sony XM6.