Backes for me. PIM really swings into his favour in my opinion. Backes also has a great perceived trade value that you might be able to take advantage of, an advantage that Purcell doesn't have. Personally I'm not big on Purcell, not sure he can repeat the numbers he had last year, but obviously he has awesome linemates that could make him better last years totals.
Gun to my head, I'd say Backes out points Purcell this season, and that's not factoring the big difference in PIM.