Two LibreOffice versions installed, not sure which to uninstall

"Best" is a matter of opinion. You have two great options.

If you prefer Deb packages, keep the upstream version.

If you prefer Snap packages, keep the Ubuntu version.

If you don't care which package, try one...then the other... until you do have a preference. Or until you forget to switch again. Or flip a coin.

There is no "best" version to uninstall. You keep the version you want to use.

• The APT version is the one that is installed by default on a default full Ubuntu install. It will receive minor bug fixes throughout the support time of your distribution. So for a stable experience, go with this.

• The Snap version was installed by yourself. It will tend to be updated to newer LibreOffice versions in the future. If you want the latest version, you may prefer the snap.