For the macOS version, if you want to mod the game (change graphics, etc.), or have it be portable on a USB drive, with all save data on the USB drive, go with the "portable" version. If you want to just play the game, and have it fully installed on your system with the app in the Applications folder, without portability, go with the "installable" version.
The Windows version is portable, since there's no Windows installer version yet. And the Linux Flatpak package isn't portable, it fully installs onto your system.