I don't have any experience with the A40, but I can say that I am absolutely satisfied with the SteelSeries and would buy it again in a heartbeat.
It's very comfortable and for the fact that it's quite closed, it doesn't warm the ears too much.
I wear it for several hours at a time while gaming or at the home office. Meetings lasting 4 hours at a time happen repeatedly and the headset has never bothered me.
However, even without active sound (i.e. when there is no sound source), it muffles the ambient noise somewhat. That took some getting used to at first.
The wireless quality and range is great (depending on what else is in between over several rooms and floors). I'm not a wireless fan per se, but I would never go back to cable headsets.