+/- is a terrible statistic. Considering Bergeron was on the most productive offensive line on the team with the best goal differential in the league, it's no surprise that he tops the charts (with many of his linemates/teammates right there with him). Two years ago Jeff Schultz lead the NHL in +/- and Ovechkin was the top forward. I think that illustrates how flawed it is in determining Selke candidates. It's much more useful as a barometer of how good a team is.
Faceoff percentage is a legitimate difference. Bergeron was a faceoff monster this year and deserves credit for that. Backes has the extra 153 hits as his "answer" to this stat. I'd imagine the faceoffs are more valuable, but I actually have no idea by how much in terms of something quantifiable. Anyone have any idea on research?
The minor penalties argument is interesting to me. Backes takes 1.1 penalties/60 and draws 1.1 penalties/60 so he's a wash for his team. Bergeron takes 0.4 and draws 1.0 so his net is obviously positive for the Bruins which is better.
Points, if anything, is something that favors Backes in my eyes. Yeah, Bergeron had more points, but the Bruins scored 54 more goals than the Blues. Backes lead his team in goals and points. Bergeron scored 8.5% of his teams goals and factored in on 24.6% of the goals. Backes scored 11.7% and factored on 26.2%. Plus, I'm not sure that scoring is even something that should be a factor for the Selke.
As for other areas in which Backes has an edge over Bergeron: time on ice. Backes averaged 1:25 more time on ice per game. Per game, Backes played 1 fewer second on the PK, 7 fewer seconds on the PP, and 1:32 more at even strength.
Backes was also among the best in the league at what I will call "field position." He had offensive zone starts just 46% of the time and ended in the offensive zone 52.4% of the time. Bergeron was also good, but not quite as good at 47.6% and 52.3%.
Finally, there's a large component of defense and being a defensive forward that just isn't accounted for in freely available statistics. (I'd imagine NHL teams have some more advanced metrics, at least I hope they do.) I think Backes and Bergeron are pretty close and voting should be interesting. Winning the Cup may give Bergeron that kick he needs to win it.