A lot of people let their AOA dues lapse after graduation from med school. I pay mine, but see little personal benefit in it. I think the reason to do it is to give back to an organization that helped you in the past.
I don't know anything about PKP. AOA likely holds more weight, not sure if it's necessary to be in both, but it honestly depends on your cash flow and the price of membership, because it certainly won't hurt to be a member.