Sheahan has not proven to be better than Glendening at any level. At best, Sheahan was as good as Glendening in college (I'm confident I watched more combined games by these two than anyone on this forum), as good in the AHL, and clearly a step behind at the NHL level, at this point in time.
But again, Sheahan likely has the higher upside, assuming he can reach it. Furthermore, he's two and a half years younger than Glendening.
What all this means is that Sheahan's development should continue to be cultivated in an effort to get him to reach his upside. At which point he will be a more valuable player. In the meantime, however, Glendening is better, rightfully more trusted, and should remain ahead of Sheahan on the depth chart. Not sure how anyone who has watched these two guys play at the NHL level could think otherwise. Has nothing to do with some perceived propensity by Babcock to want to "play grinders over skilled guys." Especially considering Sheahan isn't even a "skill guy."