Which is better for gaming?

The S95H. It has the lowest input lag tested and runs up to 165Hz on four HDMI 2.1 ports. The S90H caps at 120Hz, though it adds FreeSync Premium Pro, G-Sync and a Game Bar.

Is the S90H worth stepping up to for the bigger screen?

Only if you sit far enough back that 77 inches feels small. You give up 165Hz and the 4.2.2 speaker system to get 83 inches.

Which has better sound?

The S95H. Its 4.2.2-channel system with Object Tracking Sound+ moves audio with the action. The S90H uses a 2.1-channel setup with OTS Lite.

Do they have the same picture quality?

They share the Glare Free OLED panel, Ultra Viewing Angle and NQ4 AI Gen3 processor. The S95H is the brighter, faster set in reviews; the S90H adds AI Motion Enhancer Pro and Auto HDR Remastering.

Which is better in a bright room?

Both use Glare Free anti-glare coating. The S95H has the brightness edge, so it holds contrast better in daylight.

Neither has a published lifespan advantage. Both are OLED panels from the same generation, so treat them as equal and judge on warranty and size fit.

Samsung S95H OLED TV vs Samsung S90H OLED 4K TV

The S95H is the better TV for most people. Reviewers rate it 4.8 out of 5 and call it the brightest and fastest OLED they tested, with the lowest input lag and four HDMI 2.1 ports at up to 165Hz. The S90H is the same Samsung OLED platform in an 83-inch size, with a 120Hz panel and a 2.1-channel speaker system instead of 4.2.2. Buy the S90H only if you want the largest screen and sit far enough back to need it.

Choose the S95H if you want the best picture and the lowest input lag for gaming, and a 77-inch screen fits your wall.
Choose the S90H if you sit far from the TV and want the biggest OLED Samsung makes in this line.

Prices

Samsung S95H OLED TV: $1,987.99 at Macy's (last seen Nov 7)
Samsung S90H OLED 4K TV: $3,419.99 at Newegg (last seen Nov 10)
Difference: the Samsung S95H OLED TV is $1,432.00 cheaper than the Samsung S90H OLED 4K TV at the latest prices (Nov 7 and Nov 10)