The main difference between softbox and umbrella lighting is the area the light covers and the amount of control that you have over it.
A reflective umbrella will give you a broader area of light that goes everywhere for all-over fill light to help avoid any hard shadows, whereas a softbox is a more directional light modifier where the light is shone through a diffusion panel, similar to shoot-through umbrellas, and that gives a more natural through the window type lighting.