I ran both when I was younger, and will agree that USATF was quite tougher than AAU to qualify for nationals. I'm not sure how it is now, but back then AAU had more regionals AND the top 4 went to nationals, as opposed to USATF which has fewer regions and only top 3 go. As far as talent, it seems like USATF typically had more elites, but AAU had more depth (maybe cause more people qualified). Maybe the elites were more attracted to USATF since it's more official.
I would say USATF also ran their national meet a lot more efficiently than AAU. There was rarely any delays and everything was clear and organized. I personally liked AAU Nationals more because it was a lot more relaxed, laid back, and was a lot more tight knit and family friendly.
Back to the original topic, having run collegiately I look back (started AAU/USATF at 10 years old and got a couple AAU national medals) and question whether all my years there were worth it. The time and the money. I don't think it was as much about the times I ran, but more about the experience and jump on higher level training that you get going into High School that's the most advantageous. Most kids don't get high level training for local 5k's or from their middle school coach.
I think the biggest thing is not pushing your kids to train like a high school athlete when they're 10 years old because I have seen so many kids burn out, like age group national champions that don't even go on to run in college because they're tired of running. Let them enjoy those years and what ever times they run at that age is what they run. They will likely run close to the same times they will in HS without it, you're genetics aren't going to change.
Also, don't let your kids run too early. I realize this is just my opinion, but kids probably shouldn't run until their at least in the 5th grade. Don't start your kids running high mileage too early, and limit the race duration. Maybe don't have them race the 1500m until the 6th grade, or the 3000m until their in 8th grade.