Both are big beastly behemoths. However, if you're deciding between the 2, note the following pros and cons of the 2 in relation to each other.
LG V20
+ Higher-resolution display plus secondary ticker: May be useful for shortcuts and notifications without turning on the main display.
+ MIL-STD 810G rating: Great if you're the clumsy type.
+ Removable battery: May be useful if you're a heavy user and need to quickly swap batteries. It also charges quickly through Quick Charge 3.0.
+ Cameras : Huawei still has a better auto mode IMO, but the V20's manual mode is really where it shines, offering the user a plethora of controls and tools in order to capture or record the moment in clarity. Also has a wide-angle secondary camera which can be useful.
+ Headphones: No question. It's an outright beast.
- Battery life. It's not downright terrible but compared to the Mate, it's definitely a few steps back.
- Speaker: Meh. It's alright but nothing great.
Mate 9
+ Battery life: It's amazing. Very. One of the harder phones to kill in a single day.
+ Cameras: They're also very darn good. The dual sensors really do a nice job with dynamic range and you can great some great shots with it. The app is quite complex, however, and the video isn't on-par with the V20.
+ Size: Both this and the V20 are big, giant beastly behemoths, but somehow, the Mate 9 feels a little petite than its size would suggest thanks to the slightly curved back and slim bezels.
+ Speakers : Okay, they aren't the best, but they are pretty smart, with HTC-style high/low frequency balance when in portrait and an iPhone 7-like stereo setup when in landscape.
- Headphones : They're sorta mediocre, especially in comparison with the V20.
- EMUI : Much better than before, to the point where I can use it without any issue, but it still could use quite a bit of spit and polish to iron out some of the quirks.