Both organizations have their strengths. On a national scale, NWA is geared more toward operational meteorologists (i.e., NWS, broadcast meteorologists, private forecasters, etc.). AMS is geared more toward academia and researchers, but you get folks from all "sides" in both organizations. It all depends on what you want out of the organization. Locally, check to see what is available. My local AMS group has NWS forecasters, college professors, students, chasers, and enthusiasts that all attend the meetings. Presentations and discussions are usually geared so that all can participate or get something out of them.