if you can get it with stereo the sound quality is so much better. with my dc 10-13 the atc controllers seem away in the distance when they are talking to you , i tried a mates headset with stereo you swear the atc controller was beside you in the aircraft.just my opinion.
Since aircraft radios are exclusively mono, I can't image why a stereo headset would make any difference.
I think your mate just has a better headset than you have. The fact that it is stereo has nothing to do with it.
Buy the best headset you can afford. Apart maybe from some ANR models a good headset is less than the price of an hour of ME/IR instruction. If you get a partial instead of a full pass on your IR because you didn't hear an ATC instruction you pay dearly for your budget headset.
Good headsets also tend to have better microphones, ie less noisy. This means you can leave the intercom live rather than rely on the squelch and it will greatly improve your ability to communicate with your instructor/examiner (except of course when your instructor turns up with a lousy headset with noisy microphone).
For those with an older DC headset, invest 25 pounds every couple of years in new ear seals. Makes it feel and perform like new, and greatly increases their comfort.