Square because trackpads have always been that way my dude and can be set to a ratio similar to common screen sizes.
I use the Steam Controller and play with dual pads and I have to say I much prefer the circular pads, since the way I set it up is so that a full swipe of the touchpad results in a 180 degree turn in every game I use with mouse input. And circle has been better at providing a more consistent 180 output on swipes over a square where the it can result in varying lengths of travel depending on the angle you enter in at. Square leads to more room for error for the consistent predictable output I want from touchpad use. I don’t use stuff like acceleration, so consistency is really what I value.
And I also prefer circle for movement too. I’m not trying to replicate a square display experience when I’m using the touchpads. I really prefer circle for movement and for twin stick shooters over a square surface to rotate around when at the edge.
The square trackpads that have always been that way have been for people using it for slow desktop navigation as opposed to games.
If strong aim assist is present then that can outperform gyro, but that also can outperform a lot of mouse users to the point that even PC lobbies have mouse users abandoning mouse for aim assist supported inputs. So it depends on the level of aim assist. But, with aim assist removed from the equation gyro is better than joysticks and is surprisingly usable against mouse players despite the absence of auto aiming.