Both, unless I'm using my iPad, which is broken and when I plug in the headphones only one side works (already checked and it's not the headphones, they work fine on other devices).
Both the vast majority of the time I use my headphones or earbuds. I usually only wear them when I have the specific intention of drowning out other sounds/people.
The only situations where I tend to only wear one earbud in places like waiting rooms and I need to hear my name called or when I'm in a store so I can be more aware of my surroundings and hear people around me/communicate with cashiers etc.