I would consider any showcase labeled "National" as a national event with national level PG scouts and cross-checkers. The "regional" showcases don't usually have national scout coverage. But, yes, not a whole lot of difference except if you do well at a national event you will find it holds a bit more weight with the PG scouts.
As far as "Top Prospect" events: these are a bit more selective. The qualifier is usually because you received a Top Prospect at a previous event but sometimes players with no other PG showcase under their belts are invited. That is likely because they are ranked fairly high or have shown well at PG tourneys. I think the PG scouts just want some metrics on them. Consider it validation for the ranking they gave you. Of course if you attend and your metrics don't compare well to others with a similar ranking you may find your ranking drop!