Take the 5/3.
Reasons:
1. You are in div 4 and won't be able to train him up too much as you won't be able to afford it and so it doesn't matter if his potential isn't high, and a potential of 3 is still pretty good.
2. This player is 18, and will train fast, the 5 star rating unscouted players might have a higher potential, but the chance of them having a higher potential AND being 18 isn't much.
Pick the 5/3 player, train him up, and by the time he hits his cap you will have a very good player and you will have more draftees to train.