According to my HgSs guidebook that I accidentally stole from my elementary school library nearly a decade ago (long story), it says the following:
Sunday: Junior Stylist
Tuesday: Senior Stylist
Wednesday: Junior Stylist
Thursday: Senior Stylist
Friday: Junior Stylist
Saturday: Senior Stylist
There are no days where the two overlap, and the only day you cannot get a haircut is Monday.
And according to the source you provided earlier, it says "The younger of the Haircut brothers is cheaper at $300, but his service is often less satisfactory. However, the younger brother has a chance of occasionally increasing a Pokémon's friendship to a greater degree than the older brother is capable. The older brother is more reliable but more expensive at $500."
I believe this means the younger brother has a higher chance of having a positive result, thus raising the friendship more than the older brother, while the other brother has a higher chance of raising the beauty and sheen aspects of the Pokemon.
"Looks a little happier." Gives friendship of +1
"Looks happy!" Gives friendship of +5
"Looks delighted!" Gives friendship of +10
For best results, I reccomend you to save before sending your Pokemon to the brothers and resetting until you get the result you want, and it would be more worthwhile to go on the days the younger brother is there.
I unfortunately cannot find the exact percentages you're looking for, but I hope this helps to some degree. I believe it's safe to assume the percentages are similar to that of GSC.
In conclusion, go to the younger hair brother for the best friendship-related results, and soft reset until you got the result you're hoping for.