One 45 inch shoot-through umbrella, fabric at 18 inches (its nearest center point was 18 inches).

Very nearly the same and equal light quality as the softbox (both are 40 inch size at 18 inches). I would say equal light quality, esp for a reflected umbrella at same distance, but the curved shoot-through umbrella edges are not as close as the flat softbox edges at this close distance, so it acts a bit smaller. The main difference is the softbox light is more contained frontally, and the umbrella has wider side spill, and a shoot-through umbrella has extreme spill out the back side. Two thirds of the light goes out the back of a shoot-through, spilling all over the room. Up close at 18 inches however, we really don't care, but at say five feet, it would extreme to deal with (when direct subject and rear spill reflection paths may become more equal strength).

One egg from the first "1" Direct and from this "3" are enlarged below. In "3", the egg shell texture is smoothed (no shadows to show surface texture or imperfections), and especially notice how the light literally wraps around the left dark end of the eggs (more than in 1 because 3 is large, and more than in 4 and 5 because 3 is close). Imagine this contour was a nose or chin or cheek on a face. This "smooth" and "wrap around" is what soft light is, and large and close make it happen. Just open the umbrella, and set it close.

The softness smooths skin, which is good for portraits. Shadows enhance wrinkles and skin pores and blemishes, makes them more visible. Shadowless soft light hides them, no shadows to show them. Seems to me that no matter how technically and artistically great your photo may be, if it shows their wrinkles, the ladies won't like it.

Umbrellas definitely are equally as soft as softboxes. No actual difference. The size and closeness is what makes soft. If it is 40 inches size at 40 inches distance, it matters not if it is a softbox, a umbrella, a white reflector board, a white cloth diffusion screen, or a white reflecting wall surface. If the size and distance is the same, the light is pretty much the same. This size and closeness determines the angles of the multiple self-filling paths, creating the soft properties.

And this is just one light. Two umbrellas can essentailly eliminate all shadows. A second fill umbrella near the lens axis really helps minimize wrinkles.

Catchlight reflections in human eyes will look different from umbrellas and softboxes. This eye picture is with a reflected fill umbrella at 4 feet. It is not normally viewed this large, and it is hard to see the detail at any regular size. However, shoot-through makes it closer and larger to exaggarate it. A softbox catchlight is rectangular, which we imagine to look like a window reflection.