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.
I don't know about Artifactory but here are my reasons for using Nexus:
Dead simple install (and since 1.2, dead simple upgrade, too)
Very good web UI
Easy to maintain, almost no administrative overhead
Provides you with RSS feeds of recently installed, broken artifacts and errors
It can group several repositories so you can mirror several sources but need only one or two entries in your settings.xml
Deploying from Maven works out of the box (no need for WebDAV hacks, etc).
it's free
You can redirect access paths (i.e. some broken pom.xml requires "a.b.c" from "xxx"). Instead of patching the POM, you can fix the bug in Nexus and redirect the request to the place where the artifact really is.