There are many parameters towards your decision. CodeArtifact supports limited amount of package types, while Nexus is way more universal. More over, CodeArtifact can't be self hosted, if that's a current or future requirement of yours. I would recommend you to look at JFrog Artifactory as well, at the heart of the JFrog Platform, it is a fully featured, enterprise grade artifact management, built with strong metadata management, 100% automation enabled and fully hybrid.