The Aviation X was first launched in 1998 - it's now a 20+ year old design. The A20's were first launched in 2010 and was updated in 2015, so it's design is a good 10-15 years newer. The A20 has better active attenuation, so it's significantly quieter. It had new/redesigned microphones, new drivers, stronger design (yokes), and is slightly wider, so IMO, they are more comfortable. Pretty sure the battery life is also better, but don't quote me. I always keep mine in my case and never leave them in the sun, and have never replaced a pair of cushions. Your mileage may vary.