Is yoga better for preventing back pain or for treating it once it's already there?

Both: Yoga is excellent for building core muscles to support your back, and can prevent injuries. It also excellent for stretching and strengthening after an injury.

Good for both: It can help in both ways.

Both: Yoga done properly and with good alignment can both treat lower back and neck pain as well as help people keep a good strong core to protect their back. I recommend it to my patients all the time and practice yoga myself.