Which is better for gaming?

The LG B6. It has four HDMI 2.1 ports, native 120Hz, a 0.1ms response time, VRR, G-Sync and FreeSync Premium. The Samsung's HDMI version and refresh rate are not stated.

Which has better picture quality?

Both use OLED panels, so both deliver deep blacks. The LG adds Dolby Vision and its a8 AI processor, which gives it an edge for HDR movies. The Samsung's HDR formats are not specified.

Can I get a smaller screen than 65 inches?

Only from the Samsung. It comes in 48, 55, 65, 77 and 83 inches. The LG B6 is sold as a 65-inch set only.

Neither maker publishes a lifespan figure. OLED pixels can dim with long static images on both. The Samsung has a larger base of owner ratings, which gives more feedback on reliability.

Is the LG worth stepping up to over the Samsung?

Yes if you game on a current console or watch Dolby Vision movies in a dark room. The Samsung makes more sense if you need a smaller size or want both voice assistants.

Do both work with voice assistants?

Yes. The LG has Alexa built in and works with Hey Google and AirPlay. The Samsung supports Google Assistant and Amazon Alexa.

LG OLED B6 Series 65-Inch TV vs Samsung 4K OLED Smart TV

Buy the LG B6 for movies and mixed viewing: its OLED panel, four HDMI 2.1 ports and Dolby Vision support make it the stronger all-rounder for a dark room. The Samsung is the safer pick if you want a smaller screen or a simpler, widely reviewed set with Google Assistant and Alexa built in. Owners praise both for picture quality; the LG draws more praise for contrast and gaming features, while the Samsung's edge is size choice and setup simplicity.

Choose the LG B6 if you want the best contrast for movies, four HDMI 2.1 ports for consoles, and Dolby Vision and Dolby Atmos support.
Choose the Samsung if you need a 48- or 55-inch size, want both Google Assistant and Alexa, or prefer a set with a large base of owner ratings.

Prices

LG OLED B6 Series 65-Inch TV: $999.99 at Amazon (checked today)