They are both very similar. The intent of suspension is to temporarily remove access with the strong likelihood or reinstating access in the future. If an employee leaves the company, then you would delete that user. From the impacted user's perspective, the result is the same--they will not be able to access Clarizen.
Note, you can never fully delete a user from the system (this is in order to keep historical data such as timesheet and expense info). Even a user that has been deleted is possible to be restored in the future.