We use CircleCI because of the better value it provides in its plans. I'm sure we could have used Travis just as easily but we found CircleCI's pricing to be more reasonable. In the two years since we signed up, the service has improved. CircleCI is always innovating and iterating on their platform. We have been very satisfied.
I prefer CircleCI over Jenkins for the following reasons:
1. each repo has an isolated build container - no web of dependencies to manage
2. no ocean of plugins to configure - all what you need is the yaml file and, maybe, in a custom build image
3. decentralized execution of builds - no master/agents complexity