There is no yes or no answer to such a question. Both has its own purposes. While a test-series is focussed on making the student acquainted with a large variety of questions, mock tests seem to narrow down to relevant questions only. Test series can be of various types, chapter based, unit based, subject based etc. This can help in targeting a specific or broad area depending on the requirement. However mock tests are created to simulate exact exam like conditions and thus are more exhaustive. To ensure that one doesn’t get stuck in a particular question, time management is imperative and it comes through mock tests.