Charles, The light duty 7' will not grade as well as a heaver 6'. Yes a 7' would be nice, but because it is lighter per foot of cutting width, most likely will take more passes to get the road how you want it. I did some grading this past weekend on a new road that I am working on and more than a few times my 7' heavy grader blade stopped my 12k+lb 75hp tractor.
Oh, and yes a road grader blade takes more power to pull than a box blade.