If you have some scouting points left, I'd spend it to find out their OD rating. If your 7'2" guy has 1 OD it will be a hell to get it to a decent level. If he has 2 or more extra OD than the 18yo, I'd go for the 19yo but otherwise I'd go with the 18yo. You could scout PA too, but that trains pretty quickly with elastic effect of HA on bigs so shouldn't swing your evaluation too much.
From my experience as a U21 manager (and scout), I'd say for every 3 useful 18yo's there's 1 useful 19yo in general. Now your 19yo has an A+ rating compared to the A rating of the 18yo, but that's still advantage to the 18yo if you ask me.