It is good to perform harinam japa (silently in the mind). But Sri Chaitanya Mahaprabhu, Srimad Bhakti Siddhanta Saraswati Goswami Thakur, gurudev and guruvarga gave more stress for doing sankirtan because performers of sankirtan will have concentration on the object of worship due to the loud utterance of the Holy Name, and others will also hear it. All forms of devotion are to be performed with the knowledge that we are of Krishna. We should take complete shelter of Sri Krishna and do nama-sankirtan only for His satisfaction, and not for any other ulterior desires.
--- Jagad-guru His Divine Grace Srila Bhakti Ballabh Tirtha Goswami Maharaj