I normally release the player that costs tokens, but sign the player who is free. Then I immediately sell him.

I don't think I've ever kept a youth player as the one to sign using Tokens always seems to be overly priced, when you could go on auction and win a 5 star player for a few Tokens. The free player is then also a star worse in quality, so would not be good enough for my team and would require a lot of power training to get up to speed.

I do believe the youth players train quicker, but it will still need a lot of training to get them up to speed.

It takes 75 cones to earn a full star. So you are looking at developing them to get 150 cones if you are aiming for 2 stars. It is doable with power training, but I would advice if you are going to do it you do it in the first few days of the season, as opposed to now. If you got a good youth player on day 3 or 4 of the season then maybe it would be worth keeping and training them as you would have almost a full season before they lose a star. If you were to get a 4 star player tomorrow, you would only have a few days until he lost a star, so I wouldn't bother.