Softbox makes the light softer than an umbrella.
A situation where you'd use an umbrella over a softbox...hmm.
Well, for one, they are easy to take down and put away. Just close the umbrella and you're good to go.
Umbrellas are much easier to set up and transport than softboxes. I agree that that is the main advantage of using them.
As for the softness of the light, that's dependant on two things, the size of the light source and the relative distance to the subject...so a large umbrella will give you softer light than a smaller softbox.
The main advantage of using a softbox, IMO, is the control of the light. It's pointed in one direction and that's where the light is going. With an umbrella, light is spilling everywhere...and most likely bouncing off of walls and ceilings. For this reason, I prefer a softbox for small rooms and smaller groups...but if I was, for example, shooting a large group of people in a gym or outdoors...I would probably prefer the umbrellas.
HERE is a great web page where you can compare images taken with different light accesories.