I'm the developer behind Rectangle. There are a handful of features in Rectangle that aren't in Magnet, like being able to repeat shortcuts to achieve different sizes/positions, and being able to modify a lot more of the behavior of the app (there are more settings, and there are terminal commands listed on the readme of the GitHub repo). The main draw is that Rectangle is FOSS, which comes with obvious benefit.

Beyond that, Rectangle Pro (paid, closed source) is where I've put a ton more features. You can find Rectangle & Rectangle Pro at https://rectangleapp.com