Actually, we have little control over young player assignment so you may slowly learn. Generally speaking, zero token young player is usually worth for signing & trading. The other young player will be sacked. However, occasionally I will sign him if my cash balance is very low while tokens are sufficient.

For additional information, usually towards the end of the season, both young players assigned will be higher in quality. But at the same time, the HQ young player will need more tokens to be signed (say over 20). Note that any young player assigned after Day 2 of next season will generally be higher in quality (Provided that your team promoted to new league level. The higher quality just match the new level.). So I will be even less eager to use token to sign young player near the end of the season.

BTW, lower quality young players (tokens required can be as low as 12) assigned earlier in the season has more time to be trained and improve in quality. So, comparing with those assigned later in season, quality difference may not be large.

Some of the zero token young players are not bad in training even though they are not fast trainers. However, as their qualities are always far lower than my team average, I never has intention to keep them for long term.