Which is better for gaming?

The LG G5. Its 165Hz refresh rate and VRR support handle high frame rates better, and four HDMI ports let you keep consoles connected.

Which has the better picture?

Both are OLED with deep blacks. The LG's evo panel is brighter and has an anti-glare coating; the Samsung's main advantage is its much larger 77-inch screen.

Is the bigger Samsung worth stepping up to?

If your room is large and you sit far back, yes. The extra size changes the experience more than the LG's higher refresh rate does for movie watching.

Which handles a bright room better?

The LG G5, thanks to its glare and reflection free coating. The Samsung listing does not mention anti-glare treatment.

Do both support voice assistants?

The Samsung lists Google Assistant and Amazon Alexa. The LG lists built-in voice control but not those two assistants.

Neither has a clear edge in lifespan. OLED panels on both can dim over many years of heavy use; the LG's extra HDMI ports and newer Wi-Fi are the more practical long-term advantages.

LG G5 Series OLED evo 4K TV (55") vs Samsung S85F 77" OLED TV

The LG G5 is the better pick for most people: a 55-inch OLED evo panel with a 165Hz refresh rate, four HDMI ports, Wi-Fi 6E and a strong anti-glare coating. The Samsung S85F is the choice when screen size matters most, since it is a 77-inch OLED at 120Hz. The LG wins on motion, connectivity and glare handling; the Samsung wins on sheer size and its Google Assistant and Alexa support. Owners of both praise OLED contrast, and reviewers note the LG's brighter evo panel and the Samsung's more basic feature set for its class.

Choose the LG G5 if you sit close, watch sports or games, and want the smoothest motion, the most HDMI inputs and the least glare in a bright room.
Choose the Samsung S85F if you want the biggest OLED picture for a large room and mainly stream movies and shows.

Prices

Samsung S85F 77" OLED TV: $1,617.95 at Amazon (checked today)