1) a 19yo with 5 stars rating and 5 stars potential or a 18yo with 4 stars rating and 5 stars potential?
I think that it depends on the use you want to make of it: if you can train him, I believe that the 18 yo is better because in a year of training its growth is greater than the difference between 4 and 5 stars; if you want to sell him, the 19 yo will likely walk away at a slightly higher price, but it depends on the momentum
2) a 19yo with 4 stars rating and 5 stars potential or a 18yo with 5 stars rating and 4 stars potential?
I think that the first is the best case scenario in any case..potential rulez baby!