I went with the z test because the alternative hypothesis was one sided. If the alternative hypothesis had been two sided (rate is *different* between the two groups, no specified direction of the difference), you could just as well have performed the chi-squared test. The p value resulting from the chi-squared test is equivalent to the two sided p value resulting from the z test (and X^2 = z^2). However, the z test also allows (more straight forward) one sided testing. Does that make sense?