Sociology certainly looks like an easier option, but the study material available for Public Administration is far more superior and is available more easily. Take sociology only if you have done thorough study of sociology either in your graduation or out of interest. But you need very sound conceptual understanding for both. One more thing about Pub Ad is that the number of aspirants choosing it for Prelims is far more than sociology, so you actually stand a better chance in Pub Ad. But yes it is more difficult than Sociology, so make you pick.