Mono (or monaural) headsets only cover one ear and allows you to be aware of what is going on around you – useful at reception desks or similar customer service roles where you need to take calls with colleagues and be able to communicate easily with those around you.
Stereo (or Binaural) headsets are better for if you need to concentrate in a noisy environment as they minimise distractions. Choose a model with larger earcups if you work in a particularly noisy environment or want the best audio quality when listening to other types of audio, not just calls.