The reason that umbrellas are great is that they are more forgiving than the softbox. With the softbox you have to be more precise in your setup than with the umbrella. But that means you actually have greater control with the softbox especially if you use a grid on it). I tend to only use the square umbrellas as they tend to be a bit easier to control than the rounder umbrellas. If you stack one square umbrellas on top of the other (with a strobe on each), you can get the old Gowland wall of light which really makes for a nice light (about 3x6 feet). But a softbox of equal size is ever so much more precise in how you can control where the light goes.
Shot with the "Gowland wall of light".
Shot with a large soft box.
Cheers,
rfs