Personal preference and your work environment are key factors, if surround noise is an issue and you loose focus and distracted from what the caller is saying, simply block out that distraction by covering both ears with two speaker headset.
However if distraction not an issue and you intend to wear headset most of the day and to avoid the continual removal of headset to consult with your supervisor or others nearby, then the single ear is most suitable as it allows you to listen and focus from either ear. Great for busy call environments.
