Travis has full YML configuration in areas where CircleCI is slightly lacking still, which is great, but CircleCI offers more features, settings, and potential performance.Codeship
is simpler to use, you can use it entirely from their UI without modifying your Git repository at all. To some, this is important, existing Git repos need to be supported without adding files to them.
For others, and this is our situation, the opposite holds true--we'd rather keep our testing and deployment configuration along with our code in Git than managed somewhere separate. I believe this is a better approach in the long-term, especially when managing a large number of projects.
It also means most of our team members don't need to be in the CircleCI interface at all, possibly ever. They get added to the team, someone turns deployments on in CircleCI for the project, and then everyone can collaborate on making changes to the configuration, relying on Git's excellent merging abilities, rather than having multiple people potentially in the UI modifying configuration simultaneously, potentially reverting each others' changes.