Two. The written word should be as emotive and nuanced as possible to reflect the authors meaning. Two spaces indicates a longer pause between thoughts, just like a new paragraph indicates an even longer pause. Using one space limits the expressiveness of the author.
Although, i've got some rather unconventional ideas about sentence structure... e.g., Whenever ending a sentence with a quote, I put the punctuation outside the quotes. If the quote has its own punctuation, then i'll render it something like this: ...outhouse!".
It's clearer and more accurately describes my intent, imo, even though it's not accepted by any style guides.