Ansible is much easier to get into. The more complex your requirements are (complexity of your infrastructure, services, etc) the less useful it becomes IMHO. SaltStack feels much more feature-complete and much more powerful to me. Also Ansible has no dependency-graph which sometimes makes it hard to modularize your "plays". On the other hand, SaltStack needs to be setup on the minions first, whereas with Ansible you only need SSH. I myself now use Ansible to bootstrap SaltStack and let SaltStack do the heavy lifting.