According to my understandings, 24T academy player usually appear near the end of season and are higher in average quality (close to middle 4*). As they appear near the season end, they are less attractive to me. If academy player appear at start of next season, he is usually higher in quality (probably high 3* in next season, i.e. slightly better than those mid 4* players appearing near end of previous season) but cost probably 12 tokens only.
I do not have too many experience on buying token academy players. I believe signing them is a gamble. They are likely fast in training but this is not guaranteed.