Diana,
as Preston says, you get more light from the silver -- I'm seeing about 1 stop to 1.5 stops at the distances I work. The silver is a little cooler than the white but I adjust my formals in ACR anyway so it's not an issue really. The diffusion panel does soften the PLM for people portraits especially in the studio.

I've mentioned before that I prefer round catchlights and that is the primary advantage of the PLM for me although it does generate more light than my standard silver umbrellas and I like the way the light decays at the edges very softly.

Jim