Maw and PD interaction

The shields don't stack as they come from the same passive. You really don't want to build Maw over PD as an ADC as you're losing crit and AS that way. The only exception I could see making for this is if you were against a heavy magic damage team and you weren't really playing a crit AD anyway; someone like Lucian or Ezreal or even Kai'sa would be fine with Maw over the PD