tweeters- metal or soft dome?

This is a somewhat broad question. What are the particular differences between a metal, or hard dome tweeter and a soft one? If comparing the two using the same electronics and cables what differences can be expected?
With hard domes- aluminum, gold, etc what sound diferences are there. Are there various materials used for soft domes? If so what are they and what are their peculiarities?
I looked in my Guide to Home Audio by Robert Harley but could not find this addressed.