They give basically similar light, if you used them outdoors, given similar size, although it's easier to use a shoot-through a bit closer, and therefore potentially softer.
The difference is indoors. Shoot-throughs spill light everywhere - sideways, and half of it bounced straight out of the back. This reflects around the room completely out of control, but that usually ends up as a nice bit of auto-fill-in - if that's what you want. Suits basic one-light portraiture quite well.
White refelctive brollies are more controlled, with much less spill.