I can compare since GHT Sr was PPS for many years.....

Basically SQ will identify you on the manifest and will look after you in Y. Newspaper, OJ and advanced meal preference.

AC on the other hand is based on personality not status. That is if you are AC*G with a crappy attitude, your just another passenger. However if you have a great attitude they will come and assist you.

There were a few data points before where SE/MMs were greeted and give a few benefits from J class when seated in Y. Namely a bite, a desert and some free alcoholic drink