Trek 520 model changes during years
The primary difference between the 2015 and the 2018 seems to be the availability of disc brakes. The decision will depend on how much you think the disc brakes will improve your ride quality. I would take both for a test ride and see if you can really feel the difference in braking quality. The discs may give you better braking in wet or muddy conditions especially if the bike is loaded for touring. A concern may be the availability of disc brake parts if you travel in very remote areas.