Welcome to the site. I can't answer your question 
Historically, USSSA has been an organization primarily focused on kids 14u and down (although they offer events for older age groups). PG has been an organization primarily focused on HS age kids/kids of recruiting and scouting age. USSSA is trying to gain traction in the older age groups and PG is trying to gain traction in the younger age groups coming up into the "exposure" ages . Others can probably better speak to what degree USSSA is having success with those efforts in recent years.
One point being that unless your player/s is absolute cream of the crop nationally, it is probably not quite time to worry about exposure and still the time to have him play the most competitive and most fun baseball he can. So, either the top USSSA events or top PG events for the age group will likely provide that.
You have found a great resource for your journey in the coming years.