I own both, and personally recommend S9, G6 is much behind in terms of software and UX. Also depending of the variant you're getting, its bootloader is locked and you can't install Custom ROM (check for Supported Devices here: https://developer.lge.com/resource/mobile/RetrieveBootloader.dev). The only thing G6 is better for me is the durability. That phone is a brick, dropped it hard more than 10 times over years, not a single crack on screen and back side, only dents. And I like the flat screen better than S9 with the edge.

Honestly though, if you have newer options other than S9 and G6, probably just get newer phones. I believe these phones' batteries are pretty mediocre and probably would need charging mid-day.