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Just note that almost all of the replies are commenting about f ratio for photography. For a Dob that is not designed for photography the f ratio is less important. F ratio just changes the image size not the brightness of the image. By changing the eyepiece the size of the image changes to your eye.
An example is 2 scopes of 200mm diameter, one at f4 and one at f8. The focal lenghts will be 800mm and1600mm.
Both collect exactly the same amount of light (200mm diam). If you put a 20mm eyepiece in both scopes the view in the f4 scope will display objects at 1/2 the size of the f8 scope. The f4 view of extended objects will seem brighter because they are 1/2 the size. By changing the eyepiece to create the same magnification the images will be identical in brightness.
The choice between the 2 scopes depends on other factors as stated above- ease of collimation, handling, physical size etc.
hope this helps
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