Studies have yet to conclusively prove whether using a stool is better for your posture than your average office chair, there is a strong argument for this being the case.
This is because of the way each type of seating is designed. Chairs often have wide, plush seats, tall backrests, and built-in lumbar support which inadvertently encourages you to slouch, slump, or sit in a position that is simply no good for your spine.
Most stools retain the comfortable seat but strip out the backrest which encourages you to sit properly and maintain a healthy posture while you work.