As E.B.W. wrote, the 18th year is the most effective training year for a player in this game, you could quite easily get the 18yo past the skill level of the 19yo in that first year, therefore if you measured them both as 19yo's the 18yo would have been the better pick.
I think there is some things to weigh up as well. What is your purpose going forward for your Trainee? If you intend to stick with him and put 5-6 seasons into his development to make him the cornerstone of your team, go 18yo, best you can get.
If you are looking to boost up his skills and unload him to get some cash to help build your Arena or fund a competitive team, then a high potential and as high skilled 19yo is ok.
Personally I have never kept hold of a draft pick (nor have I had a decent one either) since I started. I went the route of trying to win first at the expense of everything else (as I had no idea I was going to like the game and stay with it at the time), and slipped into buying and selling to make quick money.
It took me about 2 seasons to get advice on building my Arena, Game Shape, Enthusiasm and a few other nuances of the game.
I have only been learning about trainees of late, probably from mid last season to now. There has been some great opinions/ experiences and information on this forum about other managers in regards to Drafts and Training and various schools of thought.
Personally I think if I had it all over again, I would have got myself a player or 2 to make myself able to beat Bots in important games (Away, TV & Rival), put all my money into my Arena for a season or 2, look to draft a few quality Trainees over a couple of seasons and train (and practice training) them along the way.