N is pentobarbital sodium which is a barbiturate that acts as a depressant. It causes the activity of the brain and nervous system to slow down. A large dose will effectively make the brain slow down to a point where it stops telling the body to keep the respiratory system working, and breathing ceases.
SN is sodium nitrite (notice nitrite and NOT nitrate). It is an inorganic compound with the chemical formula NaNO2. It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. Sodium nitrite is a powerful oxidizing agent that causes hypotension and limits oxygen transport and delivery in the body through the formation of methemoglobin. Clinical manifestations can include cyanosis, hypoxia, altered consciousness, dysrhythmias, and death.
I'd use N if it were available. I'm planning on using CO poisoning - carbon monoxide - for my CTB.