SA, like new roles doesnt raise player's value bro
As per your remarks, one player's training is to be reported with his value. But for diff qualities playing in, you should alwais compare the two once they reach the same cap. So a 15 m valued 4* q 49 will be surely faster in training then a 5* q54 19m one id we compare them right away;
Also, if we stop both of them at Q 55, 6*, the one with higher value will be the faster trainer. So it's very possible that the 4* could in the end be faster then the 5* once reached the same q value.
Now, the 5* has the bonus of pre-existent quality, so if you go straight for the raw q, he's the faster choiche despite training values. But if you plan on training both a SA and two more roles into your choiche, then the 4* in the long run will be more proficient: while he will gain 150-190 each time on the 30% train, the 5* even being a fast trainer wont exploit that far. Since SA and roles dont slow down training rates, by the end if the project the 4* will require less greens to achieve 3 roles, SA and the same quality of the 5*.
It took 28 greens to my ST to learn the 1on1 sa, while it took several more for the AD sa on my DC, which was very near to the 6*.