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Almost all space settlement designs featuring artificial gravity by centrifugal force use the inner side of the outer part of the torus:
Why is it that I see hardly any cylindrical-shaped artificial gravity habitation areas like this one:
Isn't the surface area under 1g of centrifugal acceleration for cylinders more than that of tori? Won't tori have variable fluctuations of gravity?
If my point is valid, then why do people suggest tori for space settlement designs?
Despite the obvious reason - a torus looks much more elegant than a barrel - I can come up with several points that favor a toroid:
Rounded shapes can withstand the pressure difference between inside and outside much more easily. Large areas with sharp corners like at the end caps of the cylinder are difficult to make really tight. The parts of the ISS are actually cylindrical, but their diameter is tiny compared to that of a rotating station.
Usually a fixed, easy to access, non-rotating part of the station is needed for thrusters, docks, experiments in micro gravity. This comes naturally with a torus, but is harder to build with a cylindrical shape.
To get the same inhabitable space, the radius of a torus is larger than the one of a cylinder. This makes the toroid rotate more slowly which helps with joints and traversing between fixed and rotating parts. Also, it reduces the curvature of the floor.
You mention the floor area at precisely 1g is larger in a cylinder. That's true, but there is no actual reason that the same acceleration is needed everywhere. Even if it varies by 10% throughout the station, I doubt that this can be felt by any of the astronauts. In your example double-toroid the inner ring has a reduced gravity, but this might even be a good thing, e.g. to have storage places where heavy objects can be moved more easily.
There has been a space colonization discussion started by Gerard O'Neill. The idea was to not colonize a planet or moon, but rather orbital space, particularly L5 or the trojan asteroids. They envisioned massive spinning colonies, miles across. They came in all designs, including torus and sphere, but most of them were cylinders. (with dome end caps) The advantage of the cylinder or sphere was efficiency of materials use. This worked well for huge structures because, with the radius of 1 to 3 miles, the rotation period was low enough to reduce the coriolis effect to negligible levels. The coriolis effect can cause a kind of sea sickness. In smaller structures, a torus, or better yet dumbbells can increase the radius for the same purpose. So that may be why you see a lot of toruses.
A spinning object is not stable if it is spinning around it's long axis. So if you imagine a colony the proportions of a long can, spinning around it's middle, after a while it would wobble and wind up spinning end over end. So that favors a torus, dumbbells, a sphere, or squat cylinder.
However O'Neill favored pairs of long cylinders attached together with beams at the poles. That way they would balance out and be able to keep spinning on the long axis.
