Starkler summed in up well. Ultimately the focal ratio would have to be one of the most important aspects when imaging. It should match the detector you are using to optimise the pixel per arcsecond which should also include your seeing conditions. There is not point trying to image at high resolution, say .25 arcseconds per pixel if you don't have the steady skies to do it. Generally, aim for the 1.5 to 3 arcseconds per pixel with any given combination.
It is true though that a 40" (1m) scope operating at F/7 and a 10" SCT operating at F/7 will capture the sky at the same speed, i.e. identical exposures would capture the same amount of nebulosity. The advantage the 40" scope has is image scale. The image will be much larger, this is purely based on focal length. 40" (1000mm) at F/7 give a 7000mm focal length, while the 10" SCT at the same focal ratio gives a 1750mm focal length - so the SCT image scale is smaller.
- A telescope with a fast focal ratio will provide a wider field of view
- A telescope with a slow focal ratio will provide a higher level of magnification
- Light gathering power is not all that important with the sensitivity of CCD chips today
There are some other interesting points, but can't remember them now.
