I have the Nikon 70-200mm f/2.8 VRii lens which I use during some Worship services (without flash). At times, I have mounted my 1.4 teleconverter which allows the lens to only be an f/4. That cuts the light by one stop. And because of it, the lens sometimes has trouble locking focus. There is more AF hunting going on. If you are planning on using the lens in low light situations, that is a factor you need to consider. Even if you are shooting stopped down, a faster lens in low light allows more light into the camera for focusing. I missed a shot because of using the teleconverter and only having f/4 to let in the light--only had a limited few seconds to get a photo I wanted but couldn't.
As Sparky mentioned, there are other factors to be weighed. A faster lens allows more bokeh when shot wide open. Of course, there are some instances when you can simply move your subject further from the background to achieve a shallower depth of field (more bokeh); however, that won't work in all situations. And because a faster lens allows more light into the camera, you can use a slower shutter speed when shooting wide open.
What type of subjects or scenes do you intend to use for the lens? That might help steer you in the right direction.
EDIT: if you are planning to use the lens for macro or stars/astronomy, quite often those are focused manually.