Personally, I think he should just stay dead, I've had enough of Spike to last me the rest of my life, but if I had to pick, I would say vampire. he doesn't deserve to be a human, I mean, look at Angel. Angel has been fighting on the side of good, trying to redeem himself for years, and still hasn't became human, Spike just doesn't deserve it yet.
Plus, Spike should definately not be a vampire slayer. It would just be too lame for Spike to be one, and anyways, wasn't Connor like the male equivalent of a vampire slayer? I mean, he is human with super strength and speed, so it could be true, right? Anyways, back on topic, also, If Joss made Spike a vampire, it would totally go against what Joss created the slayers for in the first place, for their to be a strong and powerful role which is taken by a woman, in order to show that a woman can be strong too. a male vampire slayer would totally go against this notion.