Sodium azide is significantly more toxic, you can generally CTB more reliably with smaller doses. It's a more violent and unpleasant experience than SN though which is why people tend to stray from it (especially because you can still survive from SA, meaning you suffered for nothing). There's also more of a chance for long term damage with SA if you survive opposed to SN. Ironically SN is often used as a treatment for SA poisoning since SA acts similarly to a cyanide.