Hi mate, as Dan mentioned i have tried both and i stuck with the pc360s. I found the bass on the A40s too overpowering. The pc360s are regarded as one of the best headsets you can get. If you take a look through http://www.avforums.com/forums/xbox-360/1569963-decent-headphones-xbox-360-part-9-a.html your see numerous posts regarding them.
Alot of us are currently waiting for reviews of the Tritton Warheads which are due for released soon. They will be the first trully wireless headset to work with the 360. Both headsets you have mentioned are of a similar price and well i would suggest maybe waiting to see how they turn out?.