Tbh it depends on how your going to train your players. If your willing to spend the tokens on power training then an 18 year old is the way forward as you can add stars quickly. If your only going to be training them on a daily basis (they dont gain quickly enough), not buying green packs then id go for a scout. A scout player is pretty much the finished, ready to go article, SA n all.
FWIW i wouldnt buy a scout for the first few levels (unless youve money/tokens to burn) as youll be able to win the league and champs league relatively easy. I bought two scouts last season and the season before and humped my league and won the champs league at a canter!