Which has the better picture for dark rooms?

The Hisense 75U6N. Its mini-LED backlight with local dimming dims dark areas instead of leaving them washed out, which the edge-lit Roku cannot do.

Is the Hisense worth stepping up to over the Roku?

For most buyers, yes. You get local dimming, Dolby Atmos and a fourth HDMI port. Step down to the Roku only if you value the simpler Roku interface more than contrast.

Which is easier to use?

The Roku. Roku OS keeps apps on one plain grid and the included voice remote is straightforward. Google TV on the Hisense adds more layers of recommendations.

Are either of these good for gaming?

Both are 60 Hz sets, so neither is a first choice for fast console gaming. The Hisense's four HDMI ports give you more room for a console plus other devices.

Which has better sound?

The Hisense, which supports Dolby Atmos. The Roku offers basic built-in sound with a night mode. A soundbar improves either one.

Do both support the main streaming apps?

Yes. The Roku runs the Roku Channel Store and the Hisense runs Google TV, and both carry Netflix, YouTube and Disney+.

Roku 75R8B5 75" QLED TV vs Hisense U6N Mini-LED 4K Google TV

The Hisense 75U6N is the stronger pick for most people. Mini-LED backlighting with local dimming gives it deeper blacks and better contrast than the Roku 75R8B5's edge-lit QLED panel, and it adds Dolby Atmos, four HDMI ports and both Google Assistant and Alexa. The Roku wins on simplicity: Roku's interface is easier to learn, and its voice remote is well liked. Owners rate the Hisense around 4.2 out of 5; both are 60 Hz sets, so neither suits serious gamers.

Choose the Roku 75R8B5 if you want the easiest smart TV interface and a simple voice remote, and you watch mostly bright rooms and regular streaming.
Choose the Hisense 75U6N if picture contrast matters to you, you want Dolby Atmos sound and four HDMI inputs, and you are fine with Google TV.

Prices

Hisense U6N Mini-LED 4K Google TV: $529.99 at DoorDash (last seen Nov 9)