Well, when I mean quality, I don't mean the Value of the player or the gain ability. If I had to face a team full of 5* players, 30 y.o., low value and a team with many 4* players, young and expensive, I would prefer the second.
About special abilities, we don't know if and how affects the quality and the performance - we just have theories.
I also like to buy young expensive fast trainers but I don't count on that when I 'm expecting performance or rates (but usually are good).
So, when the season ends and I see that some players will not have the quality I want for the next season, I sell them 1-2 days before the end and with the money I buy new ones next season.

That's my theory and the style I 'm playing. I 'not saying that I use the correct way. Of course I have the sources to support the progress of my fast trainers or else I would prefer to have less of them and more of some 5* veteran players - without train them, just use them in the games.