I believe that you should be seeking to achieve a combination and balance between both learning the content and completing practice questions/past exam papers. This is because these activities are intertwined in the sense that the former allows you to gain the knowledge required to be able to successfully complete an assessment task/exam, whereas the latter provides an opportunity to determine the extent of your familiarity with the content in addition to your ability to effectively manipulate and apply said content to a range of exam-type questions. Subsequently, you can identify any aspects/areas of the content (for example, a particular concept that you have covered in Modern History) that you may need to revisit in order to ensure that you have a thorough understanding of that information.
The main reason that you should probably avoid limiting yourself to either learning the content or completing practice material is that neither of those activities can maximise the effectiveness of your study strategy on their own. For instance, if you choose to solely learn the content in preparation for a trial exam, while you will likely be familiar with the concepts taught, the quality of the responses that you can develop may be impacted given that you would have not made any attempt at applying your knowledge and understanding of the content to relevant questions, whose requirements are likely to differ. Similarly, if you choose to prioritise the completion of practice material to form your preparations for a trial exam, while this can definitely be a source of learning the content as you mentioned (for example, if you discover that you made a significant error somewhere, you can revisit the relevant concept/topic accordingly), your overall knowledge and understanding of the content may be impacted in this case, despite the fact that this strategy maximises your exposure to exam-type questions. This is because you cannot possibly learn all the content from completing past exam papers, not to mention the risk that insufficient knowledge of the content presents on your performance potential.
I hope this helps!
