There is a reason we create a Desktop icon: so you won't have to wonder which one to use, since the Desktop icon is of course pre-configured with the correct one, which is the Couatl_updater.exe
The reason why there are two updaters, it's because the Couatl_updater.exe will first download the most recent Couatl_updater2.exe, and then will start it, and the Couatl_updater2.exe will do all the real update of the rest of the FSDT files.
This way, the updater can "Update itself" too...