TLDR: Just use CircleCI.
Having used both products in my last two positions I can say that while I once pined for Travis’s(then) superior documentation, that stage has passed and I now enthusiastically choose CircleCI.
CircleCI’s UI is amazing. It feels snappy and well suited to the task. I often find myself waiting for pages to load in Travis. The UI is also branch centric which better supports feature development where as Travis seems project focused and doesn’t suit my typical usage.
CircleCI’s ability to parse the test results(which I didn’t appreciate this till it was gone) is amazing. Its so nice not to have to scroll though all your tests to see what broke.
There are other better publicised benefits in the areas of running tests in parallel but for me these two details have the largest impact on my quality of life.