ABMP seems to be a little cheaper and they have some nice services, but I have chosen to stick with AMTA because I have some direct input into what they do and how they do it. Further, AMTA in my state is very active in setting up continuing ed, raising professional standards, improving workplace practices, and fighting to get it properly differentiated from sex entertainment in all areas.
AMTA also provides many volunteer opportunities that members participate in to give back to our communities and to network. I've gotten clients and some great professional contacts from volunteering as part of the AMTA Sports Massage team at Grandma's Marathon last year. The little extra I pay has more than repaid itself, just from that one event.