The former. I would hope that it is all positive and the only possible distress would be from the physical symptoms I have to endure. But with SI, who knows. I do know that I'll be thinking "I hope this fucking works" a lot throughout the process.
What would give me the most peace is knowing that eternal non-existence follows death. That my plan will succeed, and that I won't endure any symptoms beyond what I am expecting. These three things would be all I need. But none are guarantees. The first, no one knows what happens after death. The second, my chances of success are very good, but no method is 100%. And the third, well, every person's body is different so there's no telling how mine will react.