I know it's on Spanderfiles. I know it.
Spike goes through numerous incarnations of himself. He'll sort of pass out and wake up 'somewhere' and 'someone' else. The one I really remember is him being with a potentially older, richer man. He wears a collar then and, well, likes how it feels so much he goes out and buys one in Buffyverse and Xander takes a liking to it. Uhh, oh, and he thought that the man was cheating on him in that one (turns out he wasn't).
Aaaand, their actually tests, these incarnations. And in the end he makes some sort of choice, I think it involves Xander and I think it's his soul William who is the one talking to him. (Does that sound multiple-personality to anyone aside from me?).
I can probably remember more if that's not enough.
I know I can probably find it if I really look but... I sort of don't feel like searching through the endless files at Spanderfiles. They're fairly extensive, aren't they?!
So... thanks in advance?
FOUND - It's Down a Different Path by RogueSpike (Thanks squirlyk!)