First off, are you asking about the Mate 9 or the Mate 9 Pro? The Mate 9 does not offer 6GB and 128GB only the Mate 9 Pro. Two very different phones and thus makes the opinions offered very different as well.
I have the P10 Plus and absolutely love it so far! Mind you, I haven't played with the Mate 9, so I can't do a comparison of the two.
As mentioned, the P10 Plus does come with the 6GB 128GB option (and a 4GB 64GB or 6GB 256GB option as well) but I'm not sure if any carriers offer this setup or if that matters to you as you may be purchasing direct or 3rd party and not carrier. Mate 9 only offers 4GB 64GB option and Mate 9 Pro only offers 6GB 128GB option.
I've found that even though it is a 5.5" "traditional" (16/9 as opposed to the new 18/9 Samsung and LG offer) screen, it is quite small. My wife has an iPhone 6S Plus and it is significantly bigger than my P10 Plus. Even with a case on, my P10 Plus is still smaller than her iPhone 6S Plus. Of course, this only applies if you're referring to the 5.9" Mate 9 and not the 5.5" Mate 9 Pro.
Again, until the Mate 9/Mate 9 Pro option is clarified, opinions given will be all over the place.
This is my first non BlackBerry (BBOS, BB10 or Android) device and I am quite happy with it.