It depends.
Pros of the G6:
• More compact bezelless size (both phones are 5.7' however the G6 is more narrow and compact.. the V20 has the "classic" and wider 5.7')
• New taller aspect ratio (not sure if it's a pro or con)
• Wireless charging built-in
• IP-68 water resistant (however, not recommend for
ocean salt water or chlorine water in swimming pools, will corrode the metal parts)
• 100 mAh battery size difference (3300 mAh vs 3200 mAh)
• Completely flat screen so it's much easier to install
glass screen protectors
• Upgraded 13MP wide angle (as opposed to 8MP)
• Better looking, stylish, and has a shiney glass back
Cons of the G6 (versus the V20):
• No Quad DAC on the US models (only available on the International Korean variants), the V20 has it built-in
• Black bars on YouTube videos due to uncommon and mostly unsupported aspect ratio
• Smaller, more NARROW screen.. does NOT feel like a traditional and classic 5.7' screen (such as the Note 4 & V20) thus doesn't feel like you have a 5.7' screen at all
• DOWNGRADED FRONT FACING CAMERA... Significantly inferior to the V20, the front facing camera is NOT a wide angle... plus it looks HORRIBLY GRAINY and UGLY when compared to the V20 side by side (yes, I tested it myself)
• Wide angle POV on the rear camera is SMALLER than the V20 (this is to reduce and remove the distorted effect of the wide angle) but the MP is much higher therefore more detail (13MP vs 8MP)
• The main 13MP REAR camera is downgraded from the 16MP camera on the V20.. it doesn't look any better than the V20.. however they reduced the MP size for no reason (bigger MP doesn't necessarily mean better but it's still an advantage when you zoom/crop photos and has overall more detail. For example the 13MP vs 8MP wide angle made a big difference because it was less grainy...)
• Headphone jack is located on the top as opposed to the bottom like on the V20 (most prefer on the bottom because when you use headphones and place the phone in your pocket, it's more convenient and you don't have to have the phone face right side up)
• The back of the G6 is made of glass which is more fragile.. The V20 has a solid airplane craft material metal on the back. If you want a more durable phone, the V20 is the better option, however the G6 is still MIL-STD-810G compliant and is still more tough than the average smartphone
• NO LED NOTIFICATION LIGHT, the 'Always On Display' on the G6 utilizes the ENTIRE LCD backlight display which is kinda ugly and consumes more power VS the V20's smaller and more useful 2nd screen display (LG did however reduce the light bleed a lot so it doesn't look that bad really)
• No 2nd screen as mentioned earlier as opposed to the V20, therefore that means the G6 has a SMALLER screen size since the 2nd screen on the V20 gives you a little more screen...
• NO REMOVABLE BATTERY. It's not a deal breaker but the V20 will last much longer because the battery is much easier to replace since it's removable... plus you can swap fresh batteries on the go...
• MORE DIFFICULT TO REPAIR. As we all know, uni-body all glass smartphones are much more complicated and difficult to repair.. to open it, you have to use a heatgun and carefully remove the screen, thus risk breaking the screen plus the water resistance feature will no longer work after you open it. Basically it's not worth repairing.. the V20 you can remove the back cover and unscrew the back to repair the phone much easier...
• THE IPS LCD IS UGLIER AND MORE DIM. I compared the V20 and G6 display.. the G6 LCD looked more washed out, dim, and has inaccurate whites (not sure but I think LG wanted to lower the overall price so they used inferior screens. The G6 is not their "flagship", the V series is)
Note: I'm not saying the G6 sucks nor am I being biased (I am a fan of LG and BOTH phones). I did a comparison of the two because I did thought about upgrading to the G6 at one point but decided it wasn't that good of an upgrade from the V20 (Snapdragon 820 vs 821 CPU was not significantly better, nor was the 3300 mAh vs 3200 mAh battery, the camera also didn't look any better). The G6 had a lower starting pricepoint than the V20, meaning they probably had to cut corners and used inferior parts to fit people's budgets better. The V series is LG's "true flagship". A flagship is the company's BEST device offering and the V series is exactly that.
Overall, I'd get the G6 for the following:
Water resistance, wireless charging, more compact size, more beautiful looking and shiney phone, easy to install screen protectors bc of flat screen, higher and better 13MP wide angle camera
I'd get the V20 for the following:
Unique and useful 2nd screen display, superior built-in QUAD DAC audio experience, removable battery option, more durable metal build quality, higher 16 MP main rear camera, much better front facing camera with WIDE ANGLE view, easier to repair, a TRUE 5.7' inch screen because it's wider, excellent IPS LCD screen