PlanetOS vs Rectangle
Rectangle is a free, open-source window manager for the Mac. PlanetOS is a file converter that also includes window snapping, with halves, quarters, thirds and drag-to-edge, so if you install it for files you may not need a separate window app.
What's the same
Both snap windows to halves, quarters and thirds with Control + Option shortcuts, cycle sizes when you press the same shortcut again, and snap when you drag a window to a screen edge. PlanetOS adds a wheel of layouts when you hold Shift while dragging.
In PlanetOS, window snapping is off until you turn it on in Settings → Windows, and it doesn't count toward the free weekly actions.
When Rectangle is the better choice
If window management is all you want, Rectangle is free and does just that, with more layout options. PlanetOS makes sense when you also want the file converter, or need the same snapping on Windows and Linux.
PlanetOS does all of this on your computer. Your files are never uploaded, and it works without an internet connection.
Questions
Can I use both?
Yes, but turn off one of them so the shortcuts don't clash. PlanetOS lets you record different shortcuts.
Does PlanetOS window snapping work on Windows?
Yes, with Ctrl + Alt shortcuts.