I have both and there is a small difference in weight between the 9mm and 12mm, it depends on what you are looking for really, the 9 is tiny but sits really well on my neck on a 16" chain, my 12 is on an 18" chain and again looks great! My pic above of my collection shows them all quite well!
thank you so much for answering!! also, would you say that for $125, the 9mm feels substantial even though it's so small?