We did some testing, but couldn't really come up with an exact weight at which there is no curvature of cord that has just been removed from the spool.
Imitation Leather was the stiffest. This stays curved for a while (a few days worth of wearing) even with a few inches worth of beads on it.
Genuine Leather is also fairly stiff, and wants to stay coiled almost as much as the imitation leather. It also takes at least a few hours of being worn before it loses its curvature ... and that's with more than just 1 pendant hung on it.
Waxed Cotton straightens easier, but still hangs a bit curved on your display -- not too bad though. Waxed cotton is also quite durable and comfortable.
All 3 of the above will benefit from a few tugs to pull them straighter before going on your display. All 3 straighten naturally with help from body warmth when worn.
Rattail hangs straight with almost any pendant, but keep in mind that it can fray, so only works with pendants that have a smooth bail.
So for your purposes (just a pendant with a smooth bail, and no extra beads for extra weight), Waxed Cotton and Rattail are your best choices.
Regarding color: Black is the most popular color. But having more choices helps add interest to your display. You might want to give people their choice of 2 cording types & 3 colors.
Tan goes fine with silver and many gemstones, and is fairly neutral. And since Amethyst is one of the most popular stones set in silver, you might want purple for the people who want something more colorful than black or tan.