A manuscript assessment involves an assessor providing a written report looking at the overall strengths and weaknesses of your work. Unless you book a follow up session, no discussion takes place between yourself and the assessor. The cost is also based on the length of the work.
A mentorship involves a dialogue with a mentor, working together one on one on specific issues with your manuscript. The cost is based on time, and includes the mentor’s reading time. Instead of an overall reading of your manuscript, you may choose to engage a mentor to advise you on specific issues – such as a character arc, or a plot point – that you know exists in your work, but which you are stuck on.
See the ASA’s Mentorships.