Which is better for gaming?

The Samsung. It lists a 165Hz refresh rate with 4K V-Sync, which suits fast console and PC play. The TCL does not state a refresh rate, so you cannot count on high-frame-rate gaming.

Which is better for a bright living room?

The Samsung has an anti-glare coating and a Mini-LED backlight that pushes brightness. The TCL is larger but its glare handling is not specified.

Is the TCL worth stepping up to for size?

If you sit 10 feet or more back, yes. At 98 inches it fills a wall in a way 55 inches cannot. Up close, the Samsung's sharper panel and motion handling win.

Neither maker publishes a lifespan figure. Both are LED-backlit sets with similar panel technology, so expected life is close. The TCL's size makes it harder to move and mount without damage.

Do both support the same streaming apps?

Both cover Netflix. Samsung adds Samsung TV Plus on Tizen; TCL runs Google TV with Prime Video and Disney+ and supports Alexa and Siri.

Which has better contrast?

The Samsung. Its Neo QLED Mini-LED backlight is listed with infinite contrast and strong local dimming. The TCL is QD-Mini LED but the listing gives no contrast figures.

Samsung QN55QN90F Neo QLED TV vs TCL 98" QM6K QD-Mini LED 4K Google TV

These are different tools for different rooms. The Samsung QN55QN90F is a 55-inch Neo QLED with Mini-LED backlighting, a 165Hz panel and strong anti-glare, built for bright rooms, fast gaming and sharp detail at close range. The TCL 98QM6K is a 98-inch QD-Mini LED Google TV that trades refinement for sheer size, and owners rate it well at 4.5 out of 5. Buy the Samsung for picture quality and motion; buy the TCL if the wall is big and the seats are far back.

Choose the Samsung QN55QN90F if you sit close, game at high frame rates, or fight glare from windows and lamps.
Choose the TCL 98QM6K if you want the biggest possible picture in a large room and sit well back from the screen.

Prices

TCL 98" QM6K QD-Mini LED 4K Google TV: $1,797.99 at Electronic Express (checked today)