I enjoyed both, but comparing them is comparing two VERY different kinds of show, so I think you're going to get caught in style preference as much as quality for the basis of people's answers. I personally prefer The Office. I watched and enjoyed the entire run. Sure the last season or so got weaker, but even then it was a better than average sitcom. I hated Steve Carrell the entire time, so his departure actually made the show BETTER to me, but they were so desperate to replace him that I felt it diminished the show as they flailed between zany bosses.
Supernatural was a great episodic action/adventure show with a healthy dose of comedy relief for the first 3 seasons or so, then the Lucifer plot was OUTSTANDING. But after that, the show largely started repeating itself. They seemed to have no idea where to go from there, and it showed. While there were moments, the show had jumped the shark and I quit a few seasons later.