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This is a very popular mix-up in terminology. While f number creates different perception of the picture brightness, light gathering capacity of a telescope depends chiefly on it's aperture. 8" f3 and 8"f6 have the same aperture, hence same light gathering capacity. With f6 you will just see lesser portion of a nebula and surrounding sky, and possibly more fine detail. But you will not see "more nebula" in any of those telescopes - as they grabbed and transmitted the same number of photons. The picture will look sharper, brighter and more distorted with f3 though - due to different light distribution inside the telescope.
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