Both of these newer headsets use similar technology. I'm in the middle of my "30 day return window" so I wanted to do a comparison between my new S1 and the A20.

For ANR noise canceling in the noisy cockpit of my Husky, the A20 is just slightly better than the Sennheiser. I was surprised at this, as the Sennheiser website has all sorts of graphs and such showing the broad frequency range of noise the new S1 gets rid of. Note that my noise damaged hearing has nowhere near as broad a response range - - just ask my wife.

With the side window open and air burbling and blowing in the cockpit, the noise canceling advantage goes back to the S1 (after pressing the "smart update button") the S1 adapts nicely to the new noise environment settingling back into tranquility while the A20 continues to let you know, with an audible low frequency burble, that the window is open.

Audio quality is no contest, with the S1 walking away hands down. The A20 provides a muted bass-boosted overall impression while the S1 sounds far brighter with a flatter frequency response. Note that I have the high frequency boost option set to the middle position. Sennheiser provides this treble boost feature as many users are older and have some high frequency hearing loss - - like me. The net result is that music sounds better, but the real plus comes in increased intelligibility of radio reception.

I found the smaller ear cups of the A20 to be quite comfortable on my largish ears (I have a couple of Bose-X so I'm used to the size). But the S1 cups, with their larger cups provide an absolutely luxurious fit. The large cups can be repositioned in almost any direction to relieve any pressure that might be felt after hours of flying.

If you move from one aircraft to another, and need to switch from mono to stereo, the S1 has an external switch for that, along with a few other features missing on the Bose.
I'll be keeping the S1, but the A20 is very good indeed.