The cheaper one. Which means copper 99.9% of the time.
If you believe in cable material/sound difference theories, then copper is typically thought to warm and smoothen the sound opposed to silver that supposedly extend the highs and increase clarity (in general anyway, not always agreed up on), in which case copper would probably be better anyway since HD25 tend to be quite sharp in lower treble/treble area, and not something you likely want to boost.
Also, try to have the connecters to be of HD25 variety instead of the HD 600/650 ones. I find that the HD600/650 ones, although compatible, don't fit in as tightly and securely as the HD25 version due to size.