I will try to avoid significant spoilers. Both movies are recommended.
In terms of time, Knives Out was vertical. The victim was, if I remember correctly, in his eighties, and the suspects were his children and grandchildren. His mother was also involved in the story. The setting was an old house, obviously decorated to the owner’s particular taste. Inheritance was a major plot point.
It’s also pretty timeless. It could have been set decades earlier, and it will be just as enjoyable decades from now. The Agatha Christie influence, often commented on, includes the fact that Christie could easily have written it herself. The characters have cell phones, for example, but the story could work just as well without them.
Glass Onion is horizontal. The main characters are all roughly the same age, and none of them, as far as we know, have children. [Update: The governor apparently has one or more children.] One has a mother, who is clearly the most intelligent character in the movie, but he constantly ignores and shushes her, and she quickly leaves the story, probably to go get involved in something more interesting.
The story is also completely of this moment. It’s set in the early days of the pandemic lock-down, and several of the characters have jobs which didn’t exist ten years ago and may not exist ten years from now. The movie is full of name drops and cameos, all of which make sense at this moment in history, and most of which will probably need footnotes fairly soon.
The central character in Knives Out had a very long and successful career as a mystery writer. None of these people will have long and successful careers in anything, and in fairness they don’t seem to want that either. They just want clicks and likes and votes, and enough knowledge of the world to know whose names to drop when (and some of them fail pretty spectacularly even at that — like the [white] “influencer” and fashion designer who, we’re told, went on Oprah’s show and compared herself to Harriet Tubman).
Another difference between the movies, as I think about it, is that Knives Out was a mystery with some comedy elements, and Glass Onion is mainly a (very funny) comedy with mystery elements. In other words, Knives Out was Sleuth (the interior decoration was a clear homage to the earlier picture, in fact). Glass Onion is Clue. The mystery is in service of the comedy, not the other way around.
This is not a complaint, by the way — it’s a very clever way to keep a franchise fresh, similar to following Alien, a claustrophobic horror movie, with Aliens, a science fiction action movie. And, since I like comedies and mysteries about equally, a good combination is fine with me.
One thing that pleases me about both movies, as somebody who has written mystery stories, is that each introduces an element or two early on which seems ill-advised and cliché, but then it pays off by the end. I love it when a movie is smarter than I am. There’s a scene early in a Tarantino movie that switches languages in the middle of the scene, and at first I thought it was just the writer wanting to show off, but in fact there was a solid plot-based reason for the switch. I’m sure Tarantino was aware of, and had engineered, my incorrect initial assumption, and I enjoyed my being wrong as much as he did.