You could build a scenario that has the potential to have a PSA 9 valued higher then a 10. So say the base set holo cards pack fresh were of quality PSA 7-8 on average with the population of 9’s in single digits. If say only the Gyarados which isn’t the most popular card graded a 10 with a pop 1 and no other holo had been awarded that grade. On a pure collecting basis I could see the 9 being a higher value for people wanted the complete set. But that would depend on how much someone valued that 1 card over the others. But this will probably never happen.