TIMESTAMP is 4 bytes Vs 8 bytes for DATETIME.
Timestamps are also lighter on the database and indexed faster.
The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '0001-01-01 00:00:00' to '9999-12-31 23:59:59'.
The TIMESTAMP data type has a range of '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC. It has varying properties, depending on the MySQL version and the SQL mode the server is running in.
DATETIME is constant while TIMESTAMP is effected by the time_zone setting.