N is the gold standard for ctb. However, it's incredibly difficult to get. And expensive. However, it is relatively quick, and relatively painless. Also, it's illegal to obtain. Anyone buying it could get in serious trouble with the law.
SN on the other hand is relatively cheap. Only like $15 plus postage. It's also legal to have in many countries. And even if anyone gets it asking for a "please explain" they're are various uses for SN, so the buyer can just use that as an excuse. SN creates mild discomfort, such as increased heart rate, nausea, and headaches. All those can be negated however, with some add-ons. The thing that makes SN so attractive is the reversability of it. If the person taking the SN is found and "saved" in time, they will be pumped with the antidote, and be fine within a week or two.
Both are incredibly lethal. SN just takes longer than N.