I had the same question, and just got a response from Pete a few weeks ago regarding his metal choice:
"To answer your question, the copper used is an extremely high grade copper that has been cryogenically treated, which greatly improves the sonic characteristics. In my experimentation with silver, I’ve found that it’s a bit "tipped-up" in the treble region. At first listen, you can hear many details, but it becomes a bit unnatural and fatiguing, since it’s unbalanced sounding. The copper I use with cryogenic treatment is a very natural and real sounding, with deeper bass response (than silver), while still remaining balanced-sounding."
Maybe he gets it from *Cardas? In my case I kept the "other" cable that was made with silver, and returned his. Now to be fair the TWL was a fine sounding digital cable, it looked fantastic, and it was 1/3 the price of the silver one, and Pete was exceptional and professional; the customer service was outstanding. However at the end of the day some are pushing the envelope, and many have found a way to use ultra pure silver, or blend metals to get the resolution of silver while still retaining the smoothness of copper in their cables, and some are just sticking to what they are familiar with, which is fine.
*Note that Cardas does not use OCC copper, but he claims his is better, and a few others are quite satisfied with Cardas cables. So I don’t think it is as simple as "OCC sounds like this and non-OCC sounds like that" as some may claim.