Hi,
my 2 cents on this topic: as you are in your 2nd season - thus, you will still have limited training facilities and possibilities - me I would recommend you to go with the 19y old draftees.
As written before, the 19y old can reach 10 skill points out from the beginning, whereas an 18y old can only have 7 skill points as max. This means automatically that the TSP of an 18y is of course lower than tha TSP of an 19y old draftee. If you would be a long(er) term manager, than you might have the training facilities that would make sense for you to pick the 18y old prospect and train him one season more with your advanced training facilities and possibilities.
However, it is highly uncertain that you in your current possition would have the possibility to use the extra training season to get than a better 19y old player than it would be, when you take an 19y old draftee from the very beginning.