Users are authenticated (ie. they have to login with an email and password and you know who they are). Guests are not (anyone with a link can enter using any email and name they want - passwords for guests are optional).
Users have a dashboard to track all the reviews they are involved with. Guests can only login to specific reviews (no dashboard).
Users can set automated notifications for Reviews they are involved in - Guests cannot.
There is more control to manage the functionality available to users (roles). The functionality for guests is set to only allow viewing, annotation and approval actions. But note that approval actions for guests can only be set for "all guests approve" or "no guest approves". If there is only one guest approver then that works fine but if you have scenario where a client approver wants to share a link with lots of people at their organization, this typically you would set the client approver up as a user (approver) and allow the free guest access for all the other people (as non-approvers).