I think this is a form of trick question... cumming on her ass might be hard and way too much hashle when your cock is already in her mouth...
You guys have been watching way too much porn... what kind of man really wants to jerk off his cock in a front of naked girl when you have all the time on earth jerking off your little dick every day in front of your computer?
Anything else than finishing with a pussy sex sounds pretty disappointing for me to be honest... I prefer having it all the way balls deep inside a depthless pussy, sucking and massaging out that magic dust until my balls are shooting blanks...