Surely Delta doing this is good news...could it be a sign that the race to the bottom is coming to an end? My most comfortable economy flight was on an Air Canada 77L, when they were 9 abreast. We were forward of the engines from memory, and it was a very relaxed journey.
I for one would pay a little extra if I was assured of flying on a 9 abreast (rather than 10) 777, or an 8 abreast (rather than 9) 787.
As for the Airbii being quieter, my gut feeling is that seat location is as relevant a factor as aircraft type. I'd take a Boeing seat forward of the engines before an Airbus seat aft!