Mark,
I was wondering when this question would come up. Like Hoeman says, a MB just cuts a V shaped trench, whereas a plow (sort of like the left or right half of a MB) actually turns the ground over. You probably won't see farmers using a row of giant middle busters to prepare their fields for planting, but for your use (and mine), a MB might do just fine.
A plow turns the earth one direction and can put some side pressure on the back of your tractor. That could make it hard to drive straight, though I don't know if that would be a problem or not.
I think Hoeman's explanations are excellent. As he indicates, there is more than one way to go. A middle buster may not be designed specifically for garden prep, but that is what I use it for and I am satisfied with what it does in that capacity.
OkieG