Where will the youngsters at Barcelona get better match experience and better stats from. I am unsure of who to play where and if they will improve in either catagory.
Why is that a bad thing?
Because then they are registered for different teams. They act like they switched clubs when you transfer them. And you can't sell Barcelona At. players before changing them to your first team which can annoy your first team players because they think it's new competition arriving. And there is no option for them to play for both the reserve team and first team/U19 team.
Put your potential first teamers into Barcelona Ath, especially if they're in Liga Adelante, good experience.
You can - at least with R. Madrid's 'Castilla' - it's in the Team Settings: Take ctrl of Barca Ath. transfers etc etc
This.