Giles,
I don’t want to be a pain in the neck here, but, there is a fundamental difference between the two. You will have a loss of quality by that method.
Think of just one frame. You are zooming in on a picture and then blowing it up to 1080 (loss of quality) and then shrinking that blown-up picture to 720.
The other method of setting projects at 720 means: I have more pixels than required. My zooming acts like a crop and if I stay greater than the output size I will have more pixels than are required to compose the final picture.
But then I am stuck with 1280×720. Oh well, time for a Red-1.
Jeff
2-Xeon X5680 @ 3.33, EVGA SR-2 Mobo, 48GB DDR3, GTX 580 3072MB, 16TB Attached Storage, Win7, Vegas 11 x64