Depends. Are you a Sorcerer with passives that buff Lightening but don't want to use a Lightening Staff?
And, Crushing may be the better of the enchants right now depending on how much overall Penetration you have right now.
In other words, it all depends. Me, I would test each one by swapping out Purple Glyphs until you find the one that gives you the most DPS, then make that in Gold.