I was torn among these too and also custom art fibae 7u was on my radar, but I'm glad I went with the m9, I couldn't imagine a better timbre and technicalties for my preferences, I like the other iems I have too, but if I have to choose only one pair to live on I would have no doubts.
I bought another set of deep mount in L size, the fit in general is even better, with M size I felt the left iem sliding out, but yesterday with L size I had the feeling they were too big, I don't know how it's possible, I must have very weird ear canals