This is not a Sleep Inn vs Comfort Suites issue. I wouldn't really say "I'll never stay with Sleep Inn brand" because every franchised out hotel is a different operation.

This is a choice hotel group issue - mostly all hotel brands in this group overbook and will have no problems leaving you out cold once full. I've had this happen with Rodeway, with Quality Inn etc.

With Choice Hotels, I always have a backup idea of where I might stay, plus I also make sure to call the hotel by 4-5pm to re-confirm my reservation and tell them "hey I'll be arriving late don't give up my room" (this won't be relevant in OPs case).