For those who play with the 35T Burges like me, i think is always a good option to keep the U21, the free one can always be sold. The other are normally good or fast trainer, so can easily play in your team, or can be trade for money i think good.
You pay 24T for a player, is market value is almost the same it would cost go to the bank change tokens for money, If you get them at The beggining o season, you train them, and can sell them with a good income of money.