Using only one or the other will create problems. Below are some guidelines. There is a tendency for people to avoid unpleasant interactions, and sometimes managers will use written communication to avoid the discomfort of dealing face to face with staff. While written communication can play an important role in communicating about change, it should not be used for this reason alone. Below are some guidelines regarding the use of written versus oral communication.
Oral communication provides more opportunities for getting and keeping interest and attention.
Oral communication provides chances for both you and the other person to let off steam, cool down, and create a climate for understanding. It's easier to get feedback by observing body language and asking questions. Oral communication provides better opportunities to gain attention. Oral communication provides more flexibility, opportunity for emphasis, chances to listen to and remove resistance, and is more likely to affect people's attitudes. Generally, it is wise to use both written and oral communication.