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.
not even robert harley can provide all possible answers to the query: what is the sound of one hand clapping?
You need to listen to the speakers to get an idea. There's alot of people who don't care for most metal domes but you need to hear the speakers. Just like anything there's good and bad of every example.
As a general rule, metals will be more extended, subjectively offer more "detail" and will be "harsher". Soft domes are thought to be less agressive, the best not losing out on "detail". BUT, there's a number of variables that ultimately determine how good the system sounds; cross-over point, phase matching with the mid, etc, etc.

Is this a general question or are do you have a specific project in mind?