For moving on, when you get to the point where you are truly in control of the plane then you should be able to handle something with more performance. That means that full throttle flying seems slow to you, you are recovering from stalls before the planes self-righting characteristics do it for you, you are putting the plane where you want to on demand, and you are doing precise landings. The P-51 will be faster and it won't protect you from beginner flying mistakes like the Champ will.
As for joining AMA, for park flyers it gives you an insurance policy and a large organization to advocate for you with the government. I think everyone who cares about the hobby should join just for the second part, but there are plenty who would rather enjoy the AMA's work on their behalf without paying for it. If you don't have any kind of personal liability policy, an AMA membership will cover you if you wreck your plane into someone's car or hurt someone at the school's property.