For back pain relief, an ergonomic chair is generally better than an exercise ball.
If long periods seated at a desk are giving you a sore back, your office chair may need adjusting to give you better back support - or you may even need a new chair designed to reduce back pain. Sitting on a fitness ball for office work is not recommended.
To stay upright on a fitness ball, you have to make constant, small adjustments in muscle tension and weight distribution. This effort helps you achieve the benefits of core-strengthening exercises performed with a fitness ball. Prolonged balancing on a fitness ball during a full day of work, however, may lead to increased fatigue and discomfort in your back.