Inspector is all about security assessments of AWS based applications and their configurations against known vulnerabilities. GuardDuty is all about continuously and automatically process different foundational data sources such as CloudTrail event logs, VPC flow logs and DNS logs to find potential security threat over an entire AWS account not just only with applications and it also uses threat intelligence feeds, such as lists of malicious IP addresses and domains, and machine learning to identify unexpected, potentially unauthorized, and malicious activity within AWS environment. So as far as assessment is concerned Inspector is the right answer.