We are using Maven for a large build process (> 100 modules). We have been storing our external dependencies in source control, and using that to update a local repo.
From a learners point of view I note some specific differences between the two.
Sonatype .war deployment is not supported on Jboss application server at the time, although it does run under Tomcat.
Sonatype does not offer me an Amazon Machine Image (AMI), at present, that I could quickly stand up and test.
An Artifactory AMI is provided by Bitnami and takes a only a few minutes to stand up and a few more minutes to configure, maybe several tens of minutes dependant upon what you're trying to achieve.
Artifactory offer a SaaS version of Artifactory in the cloud so you can focus on getting things done rather than infrastructure.
I've no experience with Nexus but I've found Artifactory very intuitive and easy to configure, at least initially.
Added - I do note that the Artifactory User Guide, which may be OK for a seasoned pro, is a bit light on for some in depth explanations. For instance, starting out, one unzips and then addes a Repository, say RedHat's Jboss EAP Enterprise Repo. All goes fine but then when I tried to view the artifacts that were imported Artifactory reports zero artifacts? No errors or warnings so I'm now looking for an explanation. Is this normal or not normal? A simple explanation in the doco can quickly point one in the right direction. Being a good contributor I'm adding these comments to the project for the benefit of other starters.