I personally pay for Codeship for my own projects, and we pay/use CircleCI at up.co. I love both. Both have given us pain in random edge cases where things just don't work, which is frustrating. Both have provided amazing support whenever we needed it. Both have all of the technical needs we require. Both accommodate the exact same Github-based workflow with Heroku & Hipchat.

My suggestion: flip a coin. Or choose based on something arbitrary like which website design you like more. They are both great startups that deserve your business.