Calendar: iPad wins
Usability is again the reason that iPad's Calendar app beats Metro's Calendar app. All the controls are available onscreen, and you can very quickly see the status of outstanding invitation on the iPad.
The iPad's list view is a particularly handy view unavailable in Metro. Plus, the controls to jump to today's events, add an event, search events, and switch calendar views don't require opening a menu, as they do for Metro's calendar. A nice touch is the ability to specify the time zone for an event as you add it, which helps when you're on the road and don't want to risk miscalculating an event's time.
The iPad's calendar does lack support complex recurring events, but so does Metro's calendar.