I tried both as well, my overall preference is the Mate 9 though. But yours could be different depending on what you're looking at.
LG V20:
- one of the best audio recording I've heard so far on a phone, but speaker is not as good
- manual mode for video and camera one of the best out of the box experience
- removable battery, but battery life is average at best.
- dual sim + mSD card setup
- wide angle camera is really quite fun to play win
- QHD screen with the second display (i don't use it often, but nice for notifications)
- UI looks closer to stock android
Mate 9:
- audio recording is great too albeit a bit weaker, but speakers are much better
- better build quality
- performance is much better (faster CPU, UFS 2.1 storage)
- 4000mAh non-removable battery, but great battery life
- hybrid dual sim setup
- Monochrome camera has great quality, and has nice effects to play with
- FHD screen only, but the display quality still looks really good, even outdoors
- UI doesn't look quite as close to stock Android but tolerable to me, but the performance is top notch still.
If you want I reviewed both phones too on my personal blog.
V20
Mate 9