Glass is definitely better than plastic for longer term storage. For the longest storage (and best freshness), it's best to store whole spices whenever possible. I've got whole black peppercorns, cinnamon sticks, nutmegs, cloves, allspice berries (and probably a few more I can't think of at the moment). Of course, that requires a good spice grinder of some sort...
Glass with O2 absorbers is absolutely the best way to go... all of mine are in a dark cabinet in our cool basement, and are in mason jars.. either vacuum sealed or with O2 absorbers. Quite a bit from 1999 still are fresh and pungent when opened. I bring a small amount up and put it into small spice bottles in my kitchen cupboard, then reseal the big jar and leave it in the cellar. Even the ground spices are staying very fresh and potent under this system. I literally should be set for the rest of my life!
Summerthyme
