Here is some advice from the experts--challenges a lot of the normal ideas which originate from the film age.
Craig Stark (of PHD fame)
Stan Moore (writer of CCDStack).
Also worth remembering is that the camera pixel size plays a part. You can't just say this scope is faster than that one unless you are using the same camera. If you change a camera for one with double the pixel size and use the same scope, each pixel will collect 4x as much light in the same time. It's a bit like asking which telescope magnifies more--the answer is that it depends on the eyepiece.
Geoff