Lens For Shallow DOF

I'm buying a Canon 60D with my main interest being video with shallow DOF. I'm considering the 18-135mm or 18-200mm at this time. Both are f/3.5-5.6 . I have read that a longer focal length lens will give a shallower DOF at the minimum focusing distance from the subject which would favor the 18-200mm. This lens, however , rotates the front during focus while the 18-135mm does not. If I have a variable ND filter on the 18-200mm and refocus during a shot to bring a nearer or farther object into focus will it affect the light. Any thoughts will be appreciated.