I think different situations. Harbaugh is a former scUM player and I think this is his dream job. At one point, he restructure his contract to take less money for them to keep him and prove himself. I think Harbaugh could have hop back to the NFL if he wanted.
I'm not sure this is Day's dream job, I could see him jumping back to the NFL if the opportunity comes his way. I also think that he could jump to the NFL and even fail but get another HC job at a big time school. Day doesn't have any connections to Ohio State before he took the OC job. He basically inherited the program because of Meyer. I doubt he lose any sleep if he left Ohio State, failed in the NFL, then ended up at USC or Notre Dame.