It is better to have one prospect unsubscribe than to have them report spam. Unsubscribes are harmless to your email reputation whereas spam complaints leave a lasting negative impact. The limit for spam complaints is 1 per every thousand emails, or 0.1%.
When a prospect reports spam by clicking the "Report Spam" button, they will be unsubscribed from future emails. Pardot has a feedback loop setup for Hotmail/MSN, AOL, Comcast, Roadrunner, Rackspace, and many other common ISPs, so these spam complaint notices are captured and reported within Pardot. Once spam is reported the prospect is then opted out and unsubscribed from email communication.
Note: Gmail does not have a feedback loop so spam complaints are not reported within Pardot.