If a task can only use one core at a time, and the dual core has a higher clock speed than the quad, it will run faster on the dual. If the task can be spread out more, it will run way faster on the quad.
And these days, most CPUs can turn up the clock speed if only one or two cores is getting used--they decide how much based on their own temperature, and since the cooling is designed to cool four cores, if it only has to cool one, that one core can run hotter, and therefore faster.
Which is to say that in modern CPUs, quad is almost unquestionably faster than dual, even with single threaded tasks and if the "base" clock speed is lower.
If you're doing video editing, everything you're doing is either bottlenecked by disk speed (capture), or capable of running across many cores. Ergo, for most CPU-intensive tasks you're doing, the quad will be about twice as fast as the dual, end of story.
