I'd guess it sorta depends on whether people spend more time on Confluence or Stack Overflow. If questions are already getting answered and used, Stack Overflow

I'd guess it sorta depends on whether people spend more time on Confluence or Stack Overflow. If questions are already getting answered and used, Stack Overflow for Teams isn't likely to do better at this stage in its development. On the other hand, if questions aren't getting answered or if people forget they exist, it might be worth trying out the Stack Overflow product since developers are likely to end up there regularly. (Disclaimer: I have 0 experience with Confluence and I'm a Stack Overflow employee.)