Am using both right now.
Nexus will support more repo types with the free version than Artifactory
Artifactory will have more repos support in the paid tier
Nexus now support Conda natively (since a few weeks ago if memory serves)
Artifactory will be quite pricey, especially if you plan on having tests and staging instances to test out your config and/or custom plugins as you will have to shell for at least a Pro instance (3K+USD). Nexus will allow you to test most of these with the free version.
JFrog has exemplary support but a bit pushy with their sales. They will have help around though to get you started should you need it.
I have not tried Nexus support or sales force so I cant comment (the free version was plenty enough for our needs).
I found the plugin framework to be more usable with Artifactory though it felt a bit limited. Nexus seems more powerfull but it can be difficult to upgrade/remove plugins from a working instance. On that front I much preferred Artifactory.
Overall we did go with Artifactory, personally I find them to be pretty much equivalent. It will depend a bit on the actual technology you need supported (pypi, nuget, maven etc).