As with any A level question, it is helpful to look at the wording of the question. We are asked to compare two time periods and come to a conclusion. This is a comparison question so your answer will involve factors. When starting to plan a question like this ,it is helpful to think of three factors that are relevant to both of the periods in the question. When doing this I always have a list of factors to choose from: Government, Religion, Army, Society, Monarchy
In this case, there is one factor suggested in the wording of the question: GovernmentOur first paragraph will therefore show that the two periods were different as one had a monarchical system and the other did not. Whereas the system of rule in the interregnum period was republican (without a monarch), the personal rule of Charles the first did have a political system in which a monarch was the head of the government. Therefore the two rules were different. Straight away we already have our first paragraph.
Second paragraph: Religion A helpful hint that I always use when discussing this period is to always have a paragraph on religion, especially when I have another paragraph on politics. Choose one element of the religious policy of republican rule that is different to that of the personal rule and use it to argue that the two periods were different. An example that works really well for this question is the church hierachy in the two periods. The church hierachy of the personal rule was epeiscopalian (ie it included bishops). It was so prevailant that bishops held high offices in government. We can compare this with the acts of the republican government to ban bishops from the privy concil and then abolish them altogether.
Third Paragraph: Monarchy To get some extra marks in this section is it always usefull to add a final paragraph that says "on the other hand". This shows that you have an 'evaluative argument' which helps the markers to give you more marks. To do this, it is best to take a subject that you've already tackled and question it. So for example, we've argued that the governmental systems of the two periods were different, but how different were they? Contemporaries argued that Oliver Cromwell had the power of a king, even though he did not have the title of one. Once you have argued this you can conclude that, although Crowmwell had the power of a king, the two periods were different becuase they had a different governmental and religious structure.