Neither! There is no perfect posture if you’re going to be sitting or standing for a long period of time. Our joints, muscles, and connective tissues need a certain amount of daily motion to maintain optimal joint health. Posture is context dependent!
If there isn’t a right posture then why do I still have pain?
Your body is not prepared for the amount of the time that you spend in certain postures, or postures that you try to create. Many therapists instruct their clients to maintain a chin tuck position while sitting or standing to prevent a forward head posture. However, if your muscles or tissues are not prepared to move repeatedly or constantly into this new position, you might start to develop neck pain. Injuries occur when your actions overpower the capacity of your muscles/tissues to perform.
I’m not saying that having good posture is not important. It’s also important to be constantly moving and changing positions throughout the day. Motion is lotion!