Definitely the 18 year old.
The 19 yr old has already missed his best/fastest year of training and he can't get that back. Get 10-13 pops out of your 18 year old over the next season and then theres absolutely no comparison, by age 19 he'll be light years better then the other guy at age 19.
Subsequently, it's going to be very difficult for the 19 yr old to ever reach his full potential having missed his age 18 yr of training. So having slightly beter potential becomes a non-factor.
yes with 19 he would be better, but hey he already have a lot of training ;) And the 19 yo would be just sligthly better then the 20yo with the same amount of training and same starting skills. I think it is close betwen them, since potential is valued on the transfermarket and you definately can reach the limit of a four ball potential player even with 19. And why i mention the transfer value, since i am not a big fan of drafting for need with the vague information you have, most likelyy he won't fit and you sell him to buy a player who fit in your scheme(and if you draft for need, the answer is easy did he train big or small guys?).