Both, or preferably both plus a white reflective one - they all do different jobs.
The shoot through (translucent) umbrella can produce much softer light, mainly because it can be placed as close as you like to the subject, and partly because the light bounces everywhere, making the light source effectively much larger.
Reflective umbrellas bounce the light off of the umbrella instead of sending the light through it. Because the flash head is placed between the umbrella and the subject and because the actual light source is the umbrella (not the flash head) the light source can't be very close to the subject, so can't be very soft. But, unlike the shoot through umbrella, the light is reasonably well controlled.