Which is better for a bright living room?

The Sony's QD-OLED panel holds color and brightness better across angles, which helps in a bright room. The Samsung is also OLED, but its listing gives no brightness figures, so compare them in person if the room gets a lot of sun.

Which is better for gaming?

The Samsung's 165 Hz refresh rate beats the Sony's 120 Hz, so it suits fast console and PC play better. The Sony still handles 120 Hz and has four HDMI ports, which is plenty for most setups.

Is the 77-inch Samsung worth stepping up to over the 55-inch Sony?

Step up if room size and immersion matter most; 77 inches is a big jump over 55. Stay with the Sony if you sit close, want the best processing and sound, and don't need the extra size.

Which has better sound without a soundbar?

The Sony, because actuators behind the screen vibrate to produce sound from the picture. The Samsung listing doesn't describe its audio, so plan on a soundbar if you pick it.

Both are OLED sets with similar panel wear characteristics, and neither listing gives a lifespan figure. The Sony includes a one-year manufacturer warranty; check the Samsung's warranty terms before buying.

Do both support the same streaming apps?

Both are smart TVs with the major apps, but the Sony runs Google TV and the Samsung uses its own platform. App layouts differ, so pick the interface you find easier.

Sony Bravia 8 II 55" QD-OLED TV vs Samsung S90HA OLED TV

Buy the Sony Bravia 8 for a bright, color-rich 55-inch room where you sit close and want the best processing and sound from the panel itself. Buy the Samsung S90H if you want a much larger 77-inch picture and a faster 165 Hz panel for gaming. The Sony's QD-OLED panel and Studio Calibrated modes draw strong praise for color and motion, and its screen-actuator audio is unusually good for a flat TV. The Samsung wins on sheer size and refresh rate, but its listing gives fewer details on processing and sound, so check those before stepping up.

Choose the Sony Bravia 8 if you want a 55-inch QD-OLED with top-tier processing, wide viewing angles and sound that comes from the screen itself.
Choose the Samsung S90H if you want a 77-inch OLED and a 165 Hz panel for fast gaming in a bigger room.

Prices

Sony Bravia 8 II 55" QD-OLED TV: $1,999.99 at Walmart (last seen Nov 9)