When should I be using serverStorage and when should I use appStorage? What are the main differences? Also is there any way of telling how much of each you are using?
They work in nearly the same way: the difference is in who is allowed to access the data.
appStorage lets anyone using the app access the data. Use this for data you want all your users to share. For example, you might have a list of customers. If you put it in appStorage, all your users will be able to see the latest customer information.
serverStorage is specific to the user. Only the user who is logged in can access the data - other users of your app are not able to access it.
There is more information in the Volt Documentation.