honestly, this is tough to call then. If you're set on the other guys taking most of the time (especially the single position things like pressure and passing), then I'd go the 19 yo, because of his higher OD. He can definitely find time in the 1v1 and JS weeks you'll do. He'll need less OD and the same PA to get to a useful skill level there.
I might train the 18 year old if you can commit to fixing his OD later, when your two other trainees are finished.
Alternatively, you could flip a kroner. :)