The Jr. National and National events are definitely a more selective invitation. When we went this summer, I think there were 2 sessions and each had about 250 kids. Of those kids 2020 and 2021, at least half were committed to top teams already. I told my son while we were there that a lot of those kids would end up in the majors. First pitcher we faced was a 6'4 225lb lefty throwing 94mph. Masyn Winn was on his team and threw 98 across the infield. My son threw 89mph at the event and was in the 65th percentile. The kids there were all good. There were tons of coaches, everywhere as it was all played at Lake Pointe and all on one side. 2 days, 3 games. The Underclass All American was only about 150 kids for 2020 and 2021 combined. 8 teams of 20, only 2-3 uncommitted kids on each team. My son threw 90 and was in the 73rd percentile for the event and was in the 62nd percentile with an 89mph exit velo. The All American games (rising seniors) are only 50ish kids and there is a draft on tv for the two teams. It is played in San Diego at Petco park. The game is televised an it is a big deal. A lot of the kids that play in that event are drafted. They get custom gloves, shoes, etc. There were big lines to get in the stadium to watch the event. Lots of pro scouts attend. The Underclass All Americans get shirts, pants, shorts, a backpack and tickets to the big boy event.
I tend to think it is easier to stand out at some of the smaller PG events and get your name out there with good measurables. I know my son got invited to the more exclusive ones because he did well at the local one and was able to make the top velo and exit velo lists there. (It's harder to do at the big ones). The National Underclass showcases seem to be just bigger versions of the normal showcases so I would probably go to a showcase that is close and try to stand out there. Just my two cents.