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I look at it with respect to the field of view. Lower F Ratios provide a wider FOV which is good for large nebulae, whereas higher ratios are good for a closer view, for example galaxies.
Guiding requirements vary in both as well, with the lower ratios more forgiving on errors than longer focal length.
Hence why some people have RC's at F/10 as well as Newt's at F/4 or 5.
I personally prefer lower F Ratios.
You also have to deal with Coma at faster ratios.
I guess its upto the nature of objects you want to image, your guiding setup, pixel scale, etc.
Similar for viewing. Longer focal lengths are better for planetary viewing, and shorter ones for wider views. exact same applies to Eyepieces. 9mm's are good for planets and 20mm or above is suited for DSO's.
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