To compare the sources you should first identify the key components of the sources, eg what type of source it is. You can approach it like any other "compare" question in that first analyse the separate components (+ differences) and then at the end go through similarities. You will need to address usually the value of the sources to a historian studying xyz. So make sure you address this throughout the answer, and a judgement on the value would be ideal (they can have different values but generally everything included in the HSC exam is of value, they very very rarely include a source with no value). If it is a 12 marker a short conclusion would be good- keep in mind you need to time manage throughout the exam though.
You can also of course address the usual "perspective, reliability, usefulness" aspects but remember that those are minor components of the overarching question.