Which is better for gaming?

The LG C3. It has a native 120Hz panel, four HDMI 2.1 ports and G-Sync plus FreeSync, which suits PS5, Xbox Series X and PC gaming better than the Samsung's 100Hz panel.

Which is better for a bright room?

The Samsung S90F, because it lists an anti-glare coating. The LG C3 has no anti-glare listed, so reflections are more likely in a sunlit room.

Is the LG worth stepping up to over the Samsung?

If you have the space, yes. The LG gives you 17 more inches of screen, a faster panel and Dolby Vision. The Samsung only wins if a 48-inch set is the right fit.

Neither listing gives a panel lifespan figure. Both are OLED, so burn-in risk depends on viewing habits. The Samsung's 4.8 rating from 854 owners is the stronger reliability signal here.

Does the Samsung support Dolby Vision?

It is not listed. The LG C3 explicitly supports Dolby Vision, HDR10 and HLG, so it is the safer pick for Dolby Vision content.

LG OLED evo C3 65" 4K TV vs Samsung S90F 48-Inch OLED TV

The LG C3 is the better TV for most people deciding between these two. It is much larger at 65 inches, runs a native 120Hz panel, and has four HDMI 2.1 ports with G-Sync and FreeSync, which the Samsung's 100Hz panel and smaller 48-inch screen cannot match for movies and console gaming. The Samsung S90F is a strong OLED with an anti-glare coating and a 4.8 rating from 854 owners, and it fits rooms where a 65-inch set is too big. Choose the LG for size and gaming features; choose the Samsung for a smaller, glare-resistant OLED.

Choose the LG C3 if you want a 65-inch OLED with 120Hz, four HDMI 2.1 ports and full VRR support for gaming and big-screen movies.
Choose the Samsung S90F if your room only fits a 48-inch set and you want an anti-glare OLED with strong owner ratings.

Prices

LG OLED evo C3 65" 4K TV: $62.99 at Westside Boot and Shoe (checked today)