Both OK and technically more accurate. But as we're talking about informal language, technical accuracy doesn't signify. Come to that, as we're talking about speech, spelling doesn't signify. It's /sæk/, no matter how you choose to represent it orthographically.
b
Hi Bobk, I googled some images, "ball sack" should be the one of expression for Scrotum. Please see
ball sack - Google Search
I would say "nut sack". Why do you need to know this?