Corriander comes in several varieties. You can easily find Mexican or Moroccan corriander in the spice aisle at any grocery store. Its typically brown and wrinkled. Another form is the Indian corriander and its more yellowish and smooth.
I find the Mex/Moroc variety more black peppery and the Indian more citrusy. Over the course of almost 10 Witts, I've experimented with both varieties and find that since the sour orange peel provides what I want in the citrusy category, I prefer the additional flavor nuance of the pepperiness that the Mex/Moroc variety creates. There are some commercial Witts that seem to emmulate that pepperiness too.
Your choice. I find that an ounce of either added within 15 minutes of flameout is good.
PS: Do NOT use a regular orange from the grocery store. It really does need to be a SOUR orange peel for best effect. Most homebrew shops sell the Brewers Garden bitter or sour orange peel that is ready to go.