Three Pound,
Generally the box blade is used for heavier grading and leveling. It has teeth that can bite into the ground and its extra weight will smooth out rough ground better. On average a rear blade weighes one half or less as much as an equivalent box blade. It usually can be angled, which makes it nice for snow and moving things laterally. It generally would not have the weight for heavy grading, thus would not be as good as box blade for that purpose. A lot of tractor users use the box blade as ballast for a front end loader, and with its heavier weight, it does this well. The rear blade has the advantage of being a little easier to pull most times, if one has a smaller, or lower horsepower tractor. For example, a small SCUT tractor that weighes 1500 pounds may only be able to pull a 48" box blade, but might be able to handle a 60" rear blade for lighter grading. To each his own, but I have both and find my rear blade collects a lot of dust. If one could only choose one of them, most times the box blade is the better option.
John M