Both. Just really depends on my mood. Some days I'll play with my headphones plugged in, other days I won't. Also depends on if there's a stream I want to watch while I game or not. If there is, that obviously means I won't be using headphones.
I always have preferred headphones. Mainly because most of my games are shooters like Counter Strike: Global Offensive and Team Fortress 2. With headphones I can listen to the sound better and hear where my opponents are.
Headphones for most of the time for me. Headphones sound a lot better than speakers, they are the same surround sound - home cinema if you choose the right one's. Helps with gaming as you track and hear where your enemies more easily.