White is definitely softer, but not by as much as one would initially expect. Silver is more specular in character, but is easily tamed by use of various diffusers ... either an opal diffusion glass in tandem with a silk sock,
White Grid or Mola mesh for different degrees of softness.
I have the highly polished (almost mirror like) silver 33.5" Mola Beamm, which is even more specular than the regular silver ... the light output and versatility if this dish is quite amazing. I shoot figure competitors with no diffusion, and add a diffusion modifier for softer portraits.
Frankly, the differences between the smaller white BDs like 20.5" Profoto Soft White and 22" Mola white Demi will be there, but subtile ... but IMO as you move up in size to the 28" or 33.5" Molas, it gets more noticeable. Profoto doesn't make a silver dish, so Mola silver would be my choice for more versatile use.
- Marc
PS, here is the difference between a black or white grid:
HoneyGrids