I think this will be the story of the upcoming off season for Bills fans. The guy has taken a putrid defense and turned it into a top-10 squad (if you believe Football Outsiders, anyway) with pretty much the same personnel on the field.
But is Buffalo merely a stepping stone into a head coaching job? Like most years in the modern NFL, multiple teams will be searching for a new coach. Vikings, Bucs, Titans, Texans, Cowboys in all likelihood. Turning the Bills around has been a great opportunity for Pettine to prove that he wasn't just a stooge for Rex Ryan.
On the other hand, it's hard to miss how much the defense has gelled under his tutelage. And it's a reality that not EVERY coordinator desires a HC job. In fact, that promotion has proven time and time again to be a career setback if the first opportunity isn't a good one.
Personally I think he stays.