I am a member of both and the big difference that I see is that GOA had defended the machine gun owners better in the past, when the NRA wanted to compromise with the anti-gunners, and that the GOA sends out postcards for us to sign and stamp to send to people of political positioning to remind them of how we feel.
The NRA magazines are nice, the GOA sends out a multi-page newsletter that talks of the the political nature of what is happening. I usually send more money to the NRA as they are the most known in Washington.