The Full installer is always the very latest version of everything, code and vehicles and everything else so, by installing that one, you are sure to get all the latest updates in one go. This is also indicated on the GSX page too: the download button for the Full installer says "Contains all vehicles updates".
The vehicles updates are needed ONLY if you want to be sure you have all the latest operators with a smaller download, without downloading the full installer.
The rest of the GSX code (except vehicles) updates itself automatically from within FSX, with the live update feature.
