Size is the key. If you a a flat surface that gives off light then the larger that flat surface, the softer the light will be. The softbox keeps the light totally inside the box so there is no spill. But all things being equal, a flat reflecting surface of size x will give the same softeness as a softbox of size x, again assuming that no other light enters the equation (such as spill from the the light on the flat reflector. Keep in mind also, that we are generally talking about soft light to mean softer shadows and the softer the light the less distinct are the shadows. But the general principle is the bigger the light the softer the light (distance from the subject, naturally changes the size of the light relative to the subject). This is why a overcast day gives nice soft light and a bright sunny day gives hard light (assuming no gobos, shade, etc).
I generally use a softbox when I have lower power lights and umbrellas when I have power to spare.
Cheers,
rfs
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