Long post ahead. Tl;dr, I'll probably keep the Mate 9, but it will likely become a backup to the Pixel XL.
I got the Mate 9 three weeks ago and had been using it as my daily driver until yesterday when I received the Pixel XL I ordered back in February from the Google store (two weeks earlier than I thought).
Before the Mate 9 I was using the V20, which I've had since launch. I got the V20 to replace my Nexus 6, which now lives in my car, and was generally happy with it though I will say the speaker left a bit to be desired. I bought the V20 mainly for screen size and the fact it had Nougat out of the box. Second screen is cool, but I can live without it. Certainly not a gimmick though. Removable battery wasn't important to me nor were the quad DAC or the other advanced audio/video recording features.
I bought the Mate 9 thinking I'd give it a shot since it also had a big screen, sort of stereo speakers, Nougat and great battery life due to the 1080p screen. I was generally happy with it, but the aggressive power management (which can be dealt with but takes a lot of tinkering), the weird notification quirks, and the overly complex settings were starting to bug me. Don't get me wrong, it's a great phone, but these little things can be annoying. It's also a big phone (as is the V20, which is nearly as wide and a tad taller albeit with a smaller screen not counting the second display).
Fast forward to yesterday when my Pixel XL arrived. I know it's still the honeymoon phase, but man this is a nice phone. Yeah, the screen is smaller but with DPI adjustment it'll fit as much as either of the other two while still being readable. Battery life has also been pretty good despite all the playing around with it. Plus, and perhaps most importantly, stock Android. It's nice to not have to use Nova, mess with settings to make it like stock, and otherwise just be able to enjoy the phone. It also feels good in the hand and is much more manageable than the V20 or Mate 9. Time will tell, but I'm really digging it.
Ultimately, I think the Pixel XL will become my daily driver, with the Mate 9 as a secondary/backup phone. I'll probably sell the V20, though that is also TBD.
I'll end by saying all three phones are really good and, depending on one's individual needs, I would not hesitate to recommend any of them.
If you made it this far, thanks for reading.