SN is supposedly reversible and has no lasting damage. For it to be certain you should follow the protocol in the PPeh as far as dosage, taking anti-emetics, making sure you're not interrupted, etc. Both methods result in death by asphyxiation (lack of oxygen to the brain). Hanging might be quicker as far as when you'll lose consciousness. With SN you might experience migraine pain and it takes at least 10 minutes to lose consciousness. I'd say SN would be easier to attempt in regards to SI, which is why I would choose it.