| This is a question that is asked often and remains hard to answer. There is a lot of information and opinion out there on performing rights organizations (PROs). Have a look at that info and feel free to ask any questions. That said, the general feeling at our firm is that BMI is the preferred choice right now.
The formulas that the PROs use to determine royalties have been a topic of serious debate for many years. Because of the complicated formulas that the PROs use to determine the royalties to be paid out, the only real way to compare the two societies (at least in terms of what they pay) is if you represent 2 (or more publishers) who have a cross registered song. For example, we represent a publisher that had 50% of a song and was affiliated with ASCAP. We also represent the publisher who had the other 50% of the same song, but was affiliated with BMI. The song was successful and, when the statements came, there was a $40,000 difference between the checks paid by ASCAP and BMI for the same percentage of the same song. And no, I’m not going to say which PRO paid more ;-)
Also remember that writers affiliate with PROs too. This is particularly important when it comes to direct payment of performance royalties. For example, if your song is on the radio and the album with the song is selling, then the monies (i.e., mechanical royalties) generated by the song being on the album are paid to the publisher by the record company and then, by contract, the publisher pays the songwriters at the agreed upon percentages. However, the song playing on the radio also generates money, but 50% of that money is paid directly by the PROs to the songwriters (i.e., the PRO bypasses the publisher and sends the check directly to the songwriter.
Of course, the above examples assume the songwriter has signed an agreement with a third party publisher. If you decide to self publish, then the money obviously all goes to the same place.
Best,
MA | 
