Which is better for gaming?

The TCL QM6K. It has a 120Hz panel, VRR and a game accelerator mode. The Roku 75R8B5 is 60Hz with no gaming features listed.

Which has better picture quality in a dark room?

The TCL. Its Mini LED backlight with local dimming produces deeper blacks and stronger contrast. The Roku relies on a standard backlight, so dark scenes look flatter.

Is the Roku easier to use?

Yes. Roku OS is simpler and the included voice remote is straightforward. Google TV on the TCL offers more apps and assistant options but takes more navigating.

Does the TCL support Dolby Vision and Atmos?

Yes. The QM6K lists Dolby Vision IQ and Dolby Atmos, plus HDR10+ and HLG. The Roku lists HDR support without naming those formats.

Both should run for years, and neither brand publishes a lifespan figure. The TCL has more complex backlighting, so panel uniformity complaints are more common, while the Roku is simpler hardware.

Do I need a soundbar with either?

The TCL's Onkyo 2.1 system with a subwoofer is usable for most rooms. The Roku's built-in sound is basic, so a soundbar helps more there.

Roku 75R8B5 75" QLED TV vs TCL QM6K QD-Mini LED 4K Smart TV

The TCL QM6K is the stronger TV for most people. It has a Mini LED backlight with local dimming, a 120Hz panel (144Hz native per TCL), Dolby Vision IQ, Dolby Atmos and four HDMI ports. The Roku 75R8B5 is a plain 60Hz QLED with edge-level lighting, three HDMI ports and Roku's simple interface. Owners rate the TCL well overall, though some report panel lottery and blooming in dark scenes. Buy the Roku only if you want the Roku OS and the lowest-effort setup.

Choose the Roku 75R8B5 if you want the Roku interface, a built-in tuner and a simple 60Hz set for everyday streaming and broadcast TV.
Choose the TCL QM6K if you watch movies and sports in a brighter room, play games, or want Mini LED contrast, Dolby Vision and a 120Hz panel.

Prices

TCL QM6K QD-Mini LED 4K Smart TV: $697.99 at Abt Electronics & Appliances (last seen Nov 5)