Which is better for a bright living room?

The LG C5. Its OLED evo panel pushes higher brightness and has an anti-glare coating, so it holds up better against windows and lamps. The S90C is dimmer and reflections show more.

Which is better for gaming?

The LG C5 edges ahead with 144Hz and four HDMI 2.1 ports. The S90C also has four HDMI 2.1 ports and 120Hz with VRR, so it is still a strong gaming set, just a step behind.

Is the 77-inch LG worth stepping up to over the 65-inch Samsung?

If your room can seat you 8 to 10 feet back, yes. The extra 12 inches of diagonal and the brighter evo panel are the main reasons to pay more. If space is tight, the 65-inch Samsung is the sensible pick.

Both are OLED and share the same burn-in risk from static logos and news tickers. The LG bundle includes three extra years of protection, which makes ownership less risky. Panel life itself is similar.

Do both have the same apps?

Mostly. Both carry Netflix, Disney+, Prime Video and the major US services. LG uses webOS and Samsung uses Tizen, so the layout and a few minor apps differ.

Which has better sound?

The LG C5, with Precision Sound Master Pro and virtual surround. The S90C's built-in audio is serviceable but thin, and either set benefits from a soundbar.

LG C5 77-inch OLED evo 4K Smart TV vs Samsung S90C 65" OLED TV

The LG C5 is the better TV of the two: a larger 77-inch OLED evo panel, a newer Alpha 9 Gen8 processor, four HDMI 2.1 ports at 144Hz with VRR, and Wi-Fi 6E. The Samsung S90C is a solid older-generation 65-inch OLED with Tizen, but it is smaller and dimmer than an evo panel. Buy the LG if the room can take 77 inches and you game or watch a lot of HDR film. Buy the Samsung if 65 inches fits your wall and you want a well-regarded OLED with Samsung's app platform.

Choose the LG C5 if you want the biggest picture, the brightest OLED panel here, and full gaming features like 144Hz, VRR and four HDMI 2.1 ports.
Choose the Samsung S90C if your room suits a 65-inch screen and you prefer Samsung's Tizen platform and its track record for color and gaming.

Prices

LG C5 77-inch OLED evo 4K Smart TV: $1,796.95 at Amazon (last seen Nov 11)