1) a 19yo with 5 stars rating and 5 stars potential or a 18yo with 4 stars rating and 5 stars potential?
For me it would depend on any other information you might have, such as height, position, box score. Without any further information, probably the 19y, but I'd be happy with either.
2) a 19yo with 4 stars rating and 5 stars potential or a 18yo with 5 stars rating and 4 stars potential?
Most likely the 19y, potential is king.
The only reason I might choose the 18y is if I wanted a chance at drafting an U21 player. But in that case I'd probably draft the 19y to sell, and buy an 18y with a good chance at the U21 instead.