Rectangle dev here.
Rectangle will get you some more window actions, as you can see in the GitHub readme [1]. There are also some hidden configurations beyond what's in the UI [2].
There's also a feature called todo mode that kind of lets you pin a window on the side of the screen. This feature was created by Patrick Collison of Stripe and Nat Friedman.
And there's the fact that it's open source.
The real differences come in the paid, closed source version: Rectangle Pro [3]. Here's a comparison of some of my similar apps: https://rectangleapp.com/comparison
Rectangle is not on the Mac App Store, though, if that's important to you.
I've been using Rectangle for a while now, as the Mac equivalent for what I use on Windows, i.e. Divvy. Thanks for your hard work! I didn't even realize there was a pro version. Will look towards purchasing it.