Depending on your transfer market you can get a fast trainer with better quality for less Tokens , but Youth players have better chances to perform good. Normaly 2 out of 3 market players don't adapt well to my team and regardless of their quality get low ratings. I have to sell them after 1 or 2 seaons. Most youthplayers perform excellent and I have them on my team for 4 seasons or more.
Only my personal experience, sure there are other folks who disagree.
And when your average teamquality isn't that high (3-4*) even the free youngster can be a backup player. Even if they're no fast trainers sometimes they gain their skills quite decent. I think it's worth giving them a try before you sell.