The comments on time were really just to compare the speed at which light is gathered. Most people take lots and lots of images to stack and combine for DSOs, so you potentially end up having to take a lot more for a longer total imaging time with a longer FR.
There's no right or wrong about long and short FL's. Like a lot of things in astronomy, its just horses for courses - what things you discover you're interested in (or if you've got the money, just buy one of everything
). Short FRs are more of a modern trend, as they're usually more technically difficult to make well. Even so, shorter scopes will usually show up shortcomings in cheaper eyepieces much more than longer FL scopes.
Hope that helps and doesn't make things more confusing...
