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As Matt mentions, for the focal ratios you mention with the same focal lengths, the difference is the aperture.
But, if you have two scopes of the same aperture, but different focal lengths, the f/ratios will be different again. The terms 'fast' & 'slow' are photographic in origins, and their application to astronomical scopes is slightly different. Fast scopes are also called 'rich field telescopes', as they will see a wider FOV than a slow scope. Slow scopes tend to be more geared towards planetary applications in visual use.
There's another thing, get two scopes, same aperture, different f/ratios, but SAME magnification and the resulting image will be equally bright in both scopes. You'll need different focal length EPs, but that's all. No difference in brightness as both scopes take in the same amount of light.
Peter gives the photographic application in astronomical scopes.
Again following on from Matt, I've got a 'light bucket' 17.5" dob that's brilliant for deep sky stuff. But for the Moon & planets I love my C8. Different applications of f/ratios, aperture, optical design and resulting image.
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