I'd be interested to hear about a white ink pad that works and continues to work for the long haul. I think the white pads have a lot of pigment in them and that makes them dry out faster, just like the white ballpoint pens.
What I do to have something white stamped on black is to stamp with VersaMark and then use a fine white embossing powder for the white.
White is hard because there is no white dye ink. Whites are pigment ink and, IMO, too goopy for crisp stamping. I have SSS and HA brands and they're both goopy. I only use them for techniques where it's OK to not have a crisp image. For a better impression try Distress Oxides Picket Fence, but it's not a true white.
Like grammadee I stamp in VersaMark and heat emboss in white. I started using superfine detail embossing powder for sentiments and it really does make a difference vs regular embossing powder.