I actually did it about 2 weeks ago, I had the Korean version of the G6 with the Quad-DAC, so here goes my experience:
Why I like the LG G6:
- A lot easier to handle, I have regular size hands and the Mi Mix 2 is a bit wide, without a curved back, it doesn't feel that comfortable IMO.
- It has a headphone jack and with the Quad-DAC built in, wow it's miles better then the Mi Mix 2
- Wide angle, I miss that feature, it's just awesome taking wide angle pics
- Main Camera is just a bit better than the Mi Mix 2 in low light
Why I DON'T like the LG G6:
- Horrible selfie Cam, I'm not a big fan of selfies, but when I did take them with my kids, they were awful.
- Very slow updates
- The Korean Version has a locked bootloader without a way to unlock it.
- Lags sometimes just navigating the UI
Why I like the Mi Mix:
- Amazing design and build quality
- Very fast in day to day task (UFS 2.1)
- Good main camera (Good not great

)
- Some dev support for custom ROMs
- Quick updates, especially if you use MIUI Beta/Dev ROMs
- Very Fast charging (almost at the same level of the OP5)
Why I DON'T like the Mi Mix:
- No headphone jack, it annoys me a lot when I have to charge the phone and I can't use my earphones when it's charging.
- MIUI still does not handle notification as well as Stock Android (One example: on the lock screen, the user cannot interact with them, like you can on most other Android skins or stock)
- Mine came with Chinese version of the MIUI, so it was a bit of a pain to get the bootloader unlocked, had to install a VPN to pretend I was in China, then request the permission, then wait 72h and finally plug in to a Windows PC, not Linux or MAC, Windows only and run Xiaomi unlock tool).
- I'm on the MOKEE Rom, but there are still bugs on AOSP based ROMs =(
- GCam port still not very stable
So, these are my Pros and Cons for these two devices, now you have to see if it's worth it for you, in my opinion, I'm happy with the Mi MIX 2, the design and unlocked bootloader with some Dev support, these are big pluses for me, so I'm ok for now with the Mi MIX 2, but I do miss that Quad-DAC and wide angle camera
Hope this helped, have a good one =)