Young Link vs Toon Link differences that i've noticed:
Toon Link is shorter, mainly shorter legs (which may impact hitboxes) and seems to be floatier. Young Link will most likely be between Link and Toon Link's weight and fall speed.
Young Link has Link's classic throws, as opposed to Toon Link's decloned throws.
Young Link has a multi hit jab, while Toon Link does not.
Young Link has Link's smash attacks, while Toon Link seems to be getting at least Fsmash decloned.
Young Link's Nair is a clone of Link's kick, while Toon Link uses his sword.
Young Link has fire arrows, which presumably do residual damage as with most fire attacks, while Toon Link does not. Also, in Melee, Young Link's arrows flew a shorter distance than Link's, while Toon Link lacks this trait.
This is pure speculation, as i cant find any footage of Young Link's dash attack, but i wouldnt be surprised if he gets his dash attack updated to match Link's from Sm4sh, while Toon Link sticks with the classic dash attack that he's always had.
Most importantly: Young Link has his Lon Lon Milk taunt, while Toon Link does not, so Young Link automatically goes up at least 3 tiers over Toon Link.