BMI, ASCAP and SESAC represent both songwriters and publishers. It should be noted that even where a songwriter is represented by a third party publisher, that songwriter needs to also join a society because songwriters and publishers are paid separately by the societies. One would normally join when they have a song being performed in some medium (i.e., radio, tv, internet). As for which organization is best, each songwriter will have to decide that for themselves because it is difficult to say with certainty which society pays more. BMI, ASCAP and SESAC all have websites and will be happy to send out information brochures to interested applicants.
Choosing a performing rights society is an important decision for a songwriter to make because if a song ever becomes a hit, the performance royalties can be substantial. Thus, any serious writer should find out about affiliating with either BMI, ASCAP or SESAC.
Ben McLane Esq