I agree with Tutori.
The factors you have to consider are age, potential, height, and salary/skills.
The younger the player, the better to train, because young players train faster and you will have more time to train them. Don't bother training players over 22 unless it is for NT. For guards, the shorter the player the better (shorter players will train guard skills faster). For pf and center, the taller the better.
Potential does not affect training speed but it does affect the max of how good your player can become. So if y.ou train a player with announcer potential, he will stop improving quickly, but if you train a player with higher potentials, they can keep improving.
If you have equally young player's and equal/close potential, train the ones with the higher salaries/skills because some of your work is already done for you.