both are good,
fentanyl is a little bit more peaceful (if you manage to CTB successfully).
but you have to take tolerance, individual sensitivity and substance purity into account.
and probably use a higher dose to compensate for these factors.
but it doesn't mean SN isn't peaceful. it is as well. adverse experiences after taking SN are very minor, like nausea, vomiting etc and you'll pass out in 20 minutes anyway.
SN dose is easy to get right. it's about 25g. fentanyl dose is gonna be way more tricky because of things I mentioned.
remember that the drug cimetidine can be an useful potentiator of fentanyl. so can grapefruit juice (according to PPeH).
both of these methods have one thing in common: you cannot afford to be interrupted. both have antidotes (naloxone for fent and methylene blue for SN), so you have to do it in a place where it's unlikely that you're gonna be found too soon.