Hello!
This is a great question!
A memo is usually a shorter and more focused internal document, usually written to answer a specific question for a partner or supervisor. Therefore, you should usually get straight to the point and use a tight structure - for example, identify the issue, outline the relevant law or principles, apply them to the facts, and finish with a clear conclusion or recommendation. It is more practical and concise, and usually around a page or two (though this can differ).
On the other hand, a report is generally more formal and broader in scope. It may be created for the client (rather than internally) and is often used to explore several issues in a structured format. For a report, you are more likely to include headings, background context, and more in-depth explanations of risks and consequences. Therefore, they tend to be longer than memos (although for an assessment, they'll probably still be fairly concise!).
I hope that helps distinguish them!
marked safe from Shoosmiths PFO 
