Well, from what I hear, it is easier to join BMI. They require less to be a writer and publisher member (note: you will have to join as a writer first and then publisher if your aim is to only publish your own work.) As far as benefits, I vote for ASCAP. I'm not really sure what BMI offers, but ASCAP discounts are cool - Guitar Center discount, a music supply co. that is local for me also offers a discount to ASCAP members. Also, went to my first ASCAP member meeting last year- they treat you right - very nice din din, open bar, etc.
Overall, I picked ASCAP because they were the first PRO around and kind of have a kick ass attitude. They really strive to make sure they collect every damn cent that is owed to their writers and publishers, but I think they keep a bigger % than does BMI.
I would suggest writing/calling both and have them send you a writer membership application package (they include literature that says why they are better than the other, etc.) Compare the two and go with what feels right to you.
Brad