It's a hard question because Angel being Buffy's first love is a given. But i think inserting souled Spike in s-1 could muck up her idealized picture of Angel and the whole scoobie group dynamics.
Here is a few points in favour of Spike:
Angel indeed has that air of mystery teenage girls may like, but he probably couldn't maintain it around Spike who knows him well. With Spike hanging around Angel and the scoobies, trolling and mocking him constantly, as he did in ats-5 could Angel really play the strong and silent type?
And Spike is insightful. Buffy never could lie to him, and even when he was unsouled she talked to him, shared her problems as she couldn't to with friends as they wouldn't understand what it's like, being a slayer or with Angel who wasnt very communicative and was a bit self-centered. What if she trusted Spike, like she trusted Angel?
Also Angel was very passive in s-1. I cannot imagine Spike being stotic like Angel in Prophecy Girl, being cool about Buffy being killed and justify his inertness with going against fate is a futile endeavour. Spike is a fighter and that'd probably impress Buffy.
They also worked extremely well together, even as enemies who forced together by circumstances (becoming 2).
Young Buffy without a baggage and their history sure would like souled Spike as a comadre and perhaps more.
And as Buffyboot said, Angel is dull as a table lamp. Being presented with an alternative who is also her equal physically and intellectually but more fun it'd be a hard choice for a girl. Spike might have brought out Buffy's playful side.
Its hard for me to separate Angel's character in BTVS from the improved version of Angel in ATS whom i like, but i think both vamps were fascinating and stood a chance with Buffy.
Also both vamps are very competitive. It'd be interesting to watch a show like that, that's for sure.
