Do either of those allow supervisors to "login as" the affected account?
I haven't checked this but I would guess that a suspended account will remove students from courses whereas no login would just stop the user from logging in...?
Suspended Users: from the Forums: "Suspended users can not login, but teacher can enrol them, etc. It is a temporary prevention of login. They also do not receive any messages from moodle."
No-Log in: Simply prevents users from logging in by using the username/password directly. I assume students still get messages from Moodle with the No-Log in option.
With both options, yes, a teacher can still use the "log in as" feature to log in as the student. You can also navigate the site. However, if a student is suspended at the course level, then you cannot access those courses when using the "Log in As" feature. (Tested in 3.0)