well...no...i had 4 stars when i brought and even had two from my U21 (one was 3 star...AML, another one 4 star DC) ....
and usually i look at the value to see if he is a fast trainer or not...
from the above option A & B.... they are the same market value (1.99M) ...and for me ....A will be a slow trainer because he is higher Q but same market value as B... (well, i know that may not always true, but at least all of my players i look in that way and i never get one wrong so far) so A will be a slow trainer not a fast while B is faster than A for sure in my view... so if people want to get a 5 star right away but not going for keep the player (and also not thinking the player to develop during the season...go for "A"), if people want to keep the player and want the player develop more during the season, go for B~
btw, my U21 DC is still in my first team (7 star in LV11) , i just sold my AML age 22 last night, my AML always use as a sub but he is always 6 star in my team, i didn't put him power train to 7 stars (but he always gain 7 stars almost at the end of the season during the normal training) because i already has a very good AML at frist team, so, i kept my U21 as a sub. 2 days a ago i got a scout AML/AMC/AMR which is same as 22 years old...so i think it is the time to sell my U21 AML...as i need space...and 3 AML (+ my AMR right now can also play AML) is too much in my team...
i got a new U21 AMC in the middle of season 10 (LV10)... staring with 4 star.... i did not power train him... and now LV11. he is 5 star in LV11...