AMA is not about insurance and we've all heard that countless times. but when somebody asks about it the reply is invariably a scare story about some incident involving a claim above what HO would cover but conveniently within the limits of excess coverage provided by AMA.
Buy into AMA if you are required to as a condition for gaining access to a flying site controlled by an AMA chartered club. Buy additional liability insurance from your HO provider, or buy a PUP for whatever limit you feel comfortable with considering the greatest liability risk you are exposed to in your chosen activities. It isn't very likely to involve flying a model airplane.