Umbrella vs. Softbox
For years, I have used umbrellas and softboxes for shooting portraits, still lifes, and macro photography. When I am shooting in the studio, I prefer to use softboxes because they are easier for me to control the spill light.
When I am shooting in the field and need a large diffuse light source, I tend to use umbrellas more because they are easier to transport and faster to assemble. However, if it is a windy day, I prefer to use the softbox because it does not sail like a kit on windy days.
My largest umbrella has a 40-inch diameter. My largest softboxes have a 14-inch diameter. In general, I prefer umbrellas to softboxes because umbrellas are more cost effective. A 40-inch umbrella cost much less than a 40-inch softbox. Also, I find umbrellas more versatile. When I am shooting video, it is easier for me to use a continuous light source with an umbrella than a softbox.
All my umbrellas were made of a reflective material. I could only obtain diffuse lighting by bouncing the light off the inside of the umbrella. I recently purchased a 30-inch shoot through umbrella made of a translucent material. I can now obtain diffuse lighting by either bouncing the light off the inside of the umbrella or by shooting through the translucent umbrella. When I shoot through the translucent material, the results are like shooting with a large 30-inch softbox. The only differences are the lower cost and not being able to control the spill light.