Should I start with CYDEFENCE’s external vulnerability scanning or web application scanning?

Ideally you should do both. That being said, the majority of data breaches are caused by hackers exploiting vulnerabilities that are visible from the Internet, so you want to make sure that your external systems are protected from the outset. External scanners are also able to detect multiple web-layer issues, so if you’re new to vulnerability scanning, they are often the best tools to start with.