Hello, I've taken and overdosed many drugs over my years alive. Including Fentanyl. I felt nothing but typical opioid/opiates euphoria, it was like I was asleep when I came back personally (ignoring that I felt bodily like shit after of course).
Both are very reliable when done right and without any fight against their effects put up.
However SN isn't intoxicating the way drugs like Fentanyl are, so I suppose the question (and choice) would be if you want to be aware of you actively passing. The effects people sometimes report while live updating their ctb using SN included. The SN option shouldn't be an issue if you are certain you won't suddenly have survival instincts kick in and are alright with "leaving", I'll just say.
People already die of street Fent very often unintentionally, and quickly at that. Every second counts when trying to help someone that has overdosed of it fatally (!!!!). I'm sure one can imagine how quick it'd go with the more professional Fentanyl source you've mentioned.
Edit...... I saw someone else in this thread mention this and I feel the need to emphasize that I agree with this – if you've never taken anything like Fentanyl before, have no prior tolerance at all, you'll be out fast.