sometimes, if I want to install a program from the terminal, I get 2 options: apt and snap.
It can happen that the displayed versions are not the same, one candidate shows a higher version number than the other.
Do both libraries use the same numbering system and is it safe to assume that the higher the version number is, the newer the program is and one should only consider this variable to install new stuff?
They both use upstream version number (as in the number software developer gave to the release). They might additionally have some kind of revision number related to packaging or some patch number, but as a rule of thumb, yes, the bigger number is the most recent. If you should use that as a only variable on deciding which to install is however another discussion. Sometimes dpkg/apt version is preferred over snap regardless of version differences, for example to save a bit of disk space, but that depends on a ton of different things.