I'm not a big airbus fan but must say that I think A320 is much quieter than the B737, at least in the cockpit. I used to fly the 737 (never flown A320 except numerous jumpseats) and found the cockpit pretty loud, especially comparing to the A320 whenever I'd jumpseat. Frankly, I think Boeing did a crappy job when it comes to sound proofing the airplanes from the pilots’ perspective. That's just my take on it.