Umbrella question.

I have a 43 inch shoot-through umbrella I use with my 430 EX and Ebay Skie-Wizerds. I mainly use it as a bounce umbrella rather than shoot-through. I was wondering if the white umbrellas with the removable backing are more efficient when used in bounce fashion. I was also considering a silver umbrella for outdoor off-camera flash use, though I read in another thread the quality of light may not be desirable.
So, does the backing make the umbrella more efficient? I initially figured it would, but then I started wondering how the black would reflect any more light?

The black backing is there more for controlling spill than increasing efficiency. You want to control spill in enclosed areas, when the only light that you want bouncing around is light bouncing off the umbrella (not walls, ceilings, furniture and everything else as well).

I was interrested in a silver umbrella too; however, just about everyone I talked to said go w/ the white. I think that the stobist blog says go w/ silver if your just foing to get one umbrella so you can get more light out of you flash, but the umbrellas are cheap, why not get both and then just use the silver when you really need it.