it depends on your experience and how did you prepare? both are time constrained. I feel that CKAD should be easier as it mostly about Kubernetes development, so not the system components, services. the passing score is less and fewer exam hours. however, I did not have a go at it yet. check in medium, there are several reports e.g. https://medium.com/@ikaboubi/my-feedback-about-cka-and-ckad-e82a35585fe9

you will see that he is reporting, he studied for both at the same time, and the challenge was answering questions within the allotted time not the questions themselves.