Both phones have a lot of aspects where they are very close, with one having an edge in one area, the other in another. To me, that wasn't what made up my mind because I knew either one would be plenty good in those areas (screen, camera, software, etc). What did it for me were the features that were
exclusives, that the other phone just didn't have, period. I weighed what the S6 had that the G4 didn't, and visa versa. I thought I really wanted the fingerprint scanner on the S6, but decided in the long run, it wasn't as important as the G4's back buttons, removable battery and SD card that the Samsung lacked.
The back buttons are great for the reasons other folks have mentioned, but also because you can program them to launch features when the phone's asleep. The G4 comes preprogrammed to launch the camera with a double tap of the Volume Down button, and I installed an app that launches the flashlight with a double tap of the top button (great when picking up dog **** at 6:00 AM!) They're also just more intuitive and easier to press the right button to me than side buttons.
SD card will add storage much cheaper than Samsung, and can be removed. Extra battery can be swapped in seconds, and replaced by
you when it wears out (I HATE Apple for starting this trend). And, down the road, higher capacity batteries will come on the market...sorry Samsung

.
A minor difference that may or may not matter to you is the curve of the G4, which keeps the screen from touching the surface I put it down on. These things are
not available on the S6. All the other hardware features, other than the Samsung's Home Button/Finger Print Scanner are really comparable and very subjective. Does the Samsung feel more "premium" than a non-leather G4? Maybe, but I put a thin case on mine and now the difference boils down to the feel of the case I pick.
Weigh what the G4 has that the Samsung doesn't, and visa versa - Back buttons, removable battery and SD card, curved screen on the LG, or Home button/fingerprint scanner on the Samsung - which would be more important to you?