Jim is correct. If you are just going to use the clips as is, only smaller, then Scale to Frame Size makes the most sense. Only if you want to choose the best 1280X720 out of the 1920X1080 frame should you bother to manually set the scale using Motion.
Exporting the clip and reimporting it merely causes a generation of quality loss that is unnecessary.
Open a 1280X720 sequence, drop the 1920X1080 clip on it, right click on it to set the Scale to Frame Size and that's it. Start editing. Or use the scale parameter of the Motion effect instead. Either way, avoid exporting and reimporting whenever possible.