Is it just me, or are vampire/Slayer 'ships seriously overrated?
Spaith is just... too obvious for my tastes. They're too much alike. There's no dynamic, no contrast of personalities to be explored. It'd be like Faith doing it with herself (though I'm sure there are those who would love that as well).
Fangel is wrong, IMO. They're kindred souls. Which is to say that, while they may never have a torrid romance (like Bangel), a kinky dark relationship (like Darla/Angel), or a close friendship in which they can enjoy day-to-day life together (like Cangel), they do have this connection. And when one needs the other, they will reach out and be supportive and understanding.
And, just generally speaking, I'd rather see Faith with any number of others (Cordy/Faith is why God invented fanfiction) before she got with either of the candidates listed.